President Vladimir V. Putin of Russia took the stage in Sochi, Russia, last fall, two days after Donald J. Trump won the U.S. presidential election, and spoke of the dawn of a new world order.

“In a sense,” Mr. Putin said, “the moment of truth is coming.”

It may have already arrived.

After three years of grinding warfare and isolation by the West, a world of new possibilities has opened up for Mr. Putin with a change of power in Washington.

Gone are the statements from the East Room of the White House about the United States standing up to bullies, supporting democracy over autocracy and ensuring freedom will prevail.

Gone, too, is Washington’s united front against Russia with its European allies, many of whom have begun to wonder if the new American administration will protect them against a revanchist Moscow, or even keep troops in Europe at all.

Mr. Trump, having voiced desires to take Greenland, has pursued a rapid rapprochement with the Kremlin, while sidelining shocked European allies and publicly assailing President Volodymyr Zelensky of Ukraine.

It is a rapid shift of fortunes for Mr. Putin. He dug in on the battlefield — despite mounting pressures and costs — to wait out Western resolve in a far longer and more onerous conflict than Moscow had expected. Now, the Russian leader may believe his moment has come to shift the balance of power in favor of the Kremlin, not only in Ukraine.

“I think he sees real opportunity, both to win the war in Ukraine, effectively, but also to sideline the U.S. not just from Ukraine but from Europe,” said Max Bergmann, a Russia analyst at the Center for Strategic and International Studies in Washington who worked at the State Department during the Obama administration.

The Russian leader’s “grandiose objective,” Mr. Bergmann said, is the destruction of NATO, the 32-country military alliance led by the United States, which was established after World War II to protect Western Europe from the Soviet Union.

“I think that is right now all on the table,” Mr. Bergmann said.

The opening represents one of the biggest opportunities for Mr. Putin in his quarter-century in power in Russia.

For years, Mr. Putin has lamented the weakness Russia showed in the decade after the fall of the Soviet Union and has fixated on reversing the influence the United States has since gained in Europe at the Kremlin’s expense.

Before Russia’s invasion of Ukraine three years ago, Mr. Putin issued demands to the United States and its European allies that went far beyond Ukraine, proposing the resurrection of Cold War-style spheres of influence in a Europe divided between Moscow and Washington.

He demanded that NATO agree not to expand farther east to any nations of the former Soviet Union, including Ukraine. He also asked the United States and its Western European allies not to deploy any military forces or weaponry in the Central and Eastern European countries that once answered to Moscow.

Many of those nations, such as Estonia, Poland and Romania, have been NATO members for decades and would be difficult to defend against a Russian invasion without pre-positioned troops and equipment.

“In Putin’s view, it’s the most powerful countries that should get to determine the rules of the road,” said Angela Stent, emerita professor of government at Georgetown University. “Smaller countries, whether they like it or not, have to listen to them.”

Never mind, Ms. Stent said, that Russia lacks a superpower economy. “But it does have nuclear weapons, it has oil and gas and a veto on the U.N. Security Council,” she said. “It’s just power, hard power.”

At the time, the West immediately rejected Mr. Putin’s prewar proposals as unthinkable. The Russian leader is now almost certain to revive them in impending negotiations with Mr. Trump, a longtime skeptic of NATO and American troop presence in Europe. That has prompted a crisis among European allies, who are worried about what the U.S. president might concede.

“There is something very big going on at the moment,” said Lawrence Freedman, emeritus professor of war studies at King’s College London. “This is not business as usual. This is a very different administration, and it’s very hard to see how trans-Atlantic relations will be the same at the end of this.”

Even if Mr. Trump’s return has shifted the geopolitical environment in Mr. Putin’s favor, the Russian leader has suffered serious setbacks over three years of war, and so far has failed to achieve his goal of bringing Ukraine back into Moscow’s orbit.

Russia turned the tide on the battlefield, wresting about 1,500 square miles of land from Ukraine last year, but still has not taken the full territory of the four Ukrainian regions the Kremlin formally “annexed” in 2022. Though Ukrainian forces are reeling from personnel shortages, there has yet to be a vast Russian breakthrough causing a complete collapse of the Ukrainian lines.

Mr. Putin’s gains have also come at a significant cost. Russia is suffering losses from 1,000 to 1,500 dead and wounded per day by some estimates.

Russia’s war economy is showing strains, with 10 percent inflation, sky-high interest rates and sputtering economic growth, despite gargantuan state defense outlays. NATO has expanded to include two more nations in Russia’s backyard, Finland and Sweden, the opposite of what Mr. Putin intended.

“If you’re sitting in the Kremlin looking at this, yes, there is an opportunity, but don’t get your hopes too high,” said Thomas Graham, a fellow at the Council on Foreign Relations, who served as a top White House adviser on Russia during the George W. Bush administration. “A lot could change quickly, and at the end of the day, Trump is unreliable.”

To end the war, Mr. Graham added, both parties need to agree to stop fighting. Ukraine and its European backers most likely will not simply accept a raw deal that Mr. Trump cuts with Mr. Putin, despite intense pressure they might face from Washington.

“This is a lot more complicated than simply Putin and Trump sitting down and signing a piece of paper basically prepared by Putin,” Mr. Graham said, noting that he “wouldn’t pop the champagne corks in Moscow right now,” even if Russia appears to be in a better position than it once was.

Heading into talks, Mr. Trump faces the added difficulty that Mr. Putin is not a popular figure among the American public. Any deal seen as Kremlin appeasement could prove difficult to sell at home, though the vast majority of Americans favor a quick end to the conflict, which Mr. Trump promised on the campaign trail.

Last year, more than eight in 10 Americans expressed a negative view of Russia, and 88 percent said they did not have confidence in Mr. Putin to do the right thing in international affairs, according to a Pew Research Center poll. Nearly two-thirds of respondents called Russia an enemy of the United States.

Mr. Trump’s own secretary of state, Marco Rubio, who has been leading the talks so far, has in the past called Mr. Putin “bloodthirsty,” “a butcher” and “a monster.”

Mr. Putin, however, has benefited from changes in the information landscape and increasing admiration in the right-wing media universe, led by the former Fox News host Tucker Carlson, who visited Moscow to interview him last year.

Three years ago, Ukrainians successfully took to Twitter to popularize their cause around the globe at the outset of the invasion. But disinformation, often friendly to the Kremlin, has flourished on the platform since Elon Musk took over the company in 2022 and later rebranded the social media giant as X.

Federal prosecutors last year said they had unearthed a covert Russian campaign to spread Kremlin-friendly messages by funneling money to right-wing American influencers through a Tennessee-based media company.

The Western countries that lined up against Mr. Putin are facing their own problems at home. The two most influential countries in continental Europe — France and Germany — have been mired in political dysfunction for months and gripped by the rise of Kremlin-friendly far-right parties, now enjoying the backing of both Russian and American officials.

In the United States, Mr. Trump’s defense secretary has ordered senior leaders to begin the process of identifying major cuts in military spending. Some incoming top officials at the Pentagon have pushed for a significant withdrawal of American forces from Europe to focus on China, arguing that Europeans can handle their own defense.

Mr. Putin and his advisers would welcome the change.

“I would imagine if they are smart, they would adhere to Napoleon — when your enemy is destroying itself, don’t interfere,” Mr. Graham said. “I think that would be the approach at the moment.”

Alina Lobzina contributed reporting.

Germany’s presumptive next chancellor could be weak from the start, for all sorts of reasons. He didn’t win a particularly high share of votes. Most Germans don’t think he’ll make a great leader. Many of the people who backed his party in Sunday’s elections say they’re not enthused with him, personally.

And yet Friedrich Merz will step into Germany’s top job with an immediate chance to be the most influential German chancellor globally since the financial crisis heyday of his longtime rival, Angela Merkel.

There are two reasons for that. One is President Trump, whose threats to abandon Europe militarily and circumvent it in war negotiations with Russia have given Mr. Merz an immediate foil on the world stage.

The other is the bold, sometimes impetuous, style that has vaulted Mr. Merz to the door of the chancellorship, even as it sometimes alienated friends and foes along the way.

The day after Mr. Merz and his Christian Democrats won a high-turnout parliamentary election and the right to form the country’s next government, the big question in Germany was whether he could deliver the sort of dramatic change that voters said they were craving.

Surveys of people at the polls on Sunday showed widespread national anxieties over the country’s faltering economy, including stagnant growth and a high cost of living, and deep divisions on the hot-button issue of migration into Germany from the Middle East and elsewhere.

They also showed widespread fear among Germans that they are vulnerable to security threats from Russia and the United States, under a shifting world order stirred by Mr. Trump.

Mr. Merz started with one advantage in his quest to address those worries. The final allocation of seats in Parliament will allow him to form a two-party coalition with the left-leaning Social Democrats, and avoid a more cumbersome or fragile three-party coalition. Mr. Merz said he hoped to have coalition negotiations wrapped up by Easter, in just under two months, which would be relatively fast for the traditionally plodding German political system.

Analysts cautioned he might need to move faster, still.

“Friedrich Merz only has this one shot,” said Andrea Römmele, a dean and professor at the Hertie School, a private university in Berlin. “He has to succeed and he knows that.”

Mr. Merz is a businessman, a licensed pilot and a longtime aspirant to the chancellor job. He earned a personal fortune, first as a lawyer and a lobbyist and then as the supervisory board chairman of the German subsidiary of BlackRock, the American investment giant.

In the early 2000s, he lost a leadership struggle in the Christian Democrats to Ms. Merkel, who went on to win four terms as chancellor. He returned when Ms. Merkel announced she was stepping down, but the party suffered a surprising defeat in 2021 to Olaf Scholz and the left-leaning Social Democrats.

“It used to be he’d come home nights,” Mr. Merz’s wife, Charlotte, told a laughing crowd at a rally on Friday in the western city of Oberhausen. “Now, if I want to see him, I have to look at Instagram or turn on the TV.”

He is a man of dry wit, who sometimes appears to enjoy twisting a rhetorical knife. In Oberhausen, he paused in the middle of his closing-argument speech to repeatedly castigate Robert Habeck, the chancellor candidate for the Green Party and the current economy minister. Mr. Habeck was sitting fourth in the polls at the time.

Mr. Merz is sometimes fond of saying things that many German leaders think, but would prefer not to say in public. Increasingly, he has applied that knack to comments about Mr. Trump. Last week, Mr. Merz wondered if the United States would remain a democracy, or if NATO would cease to exist. In a televised round-table on Sunday after polls closed, he said that “it’s clear that this administration is largely indifferent to Europe’s fate.”

Analysts and fellow politicians describe Mr. Merz as a fan of bold action and brash promises, which do not always work out. He won his party leadership post in part by promising to cut voter support for the hard-right Alternative for Germany Party, or AfD, in half. Instead, the party doubled its vote share on Sunday, compared with the last parliamentary election.

In January, comfortably ahead in the polls but worried about the AfD gaining ground on him, Mr. Merz took a political gamble.

Mr. Merz, who had already pushed the Christian Democrats to the right on migration, forced a vote in Parliament on a package of tough-on-migrant measures. He knew it could only pass with votes from the AfD, which has long made migration a signature issue. The ploy broke a decades-long taboo in German politics.

Protests ensued across the country. The Christian Democrats initially appeared to have suffered little damage from it, but they ended up winning a lower vote share than polls suggested they would before the gambit.

It galvanized the traditional party of the far left, Die Linke, which took votes on Sunday in part from the more center-left Green Party.

That shift ended up hurting Mr. Merz. He was not able to form a two-party coalition with the Greens, reducing his leverage in negotiations with the Social Democrats over ministry posts and planned legislation.

He also might no longer have the votes in a new Parliament to pass a key priority — a major increase in military spending, financed by an increase in government borrowing, which would require a two-thirds majority to pass.

Mr. Merz acknowledged as much at a news conference on Monday. He also suggested a potentially creative way around the problem: working with the Greens, the Social Democrats and the Free Democrats, or F.D.P., in a lame-duck session of Parliament.

The goal would be to relax borrowing limits and accelerate the nation’s rearmament, to better prepare for the possibility of America withdrawing troops and its longstanding security guarantee from Germany. Such legislation may need to be fast-tracked before the Parliament changes and passing it gets harder.

He set a March 24 deadline for those talks, the last day the old Parliament would be active.

“I would first like to hear the views of the Social Democrats, the Greens and the F.D.P. before we come to any decisions here,” he said. We all know that the Bundeswehr” — the German military — “will need a great deal more money in the next few years. We need to talk about how we organize that.”

Diplomats and analysts expressed some optimism on Monday that Mr. Merz could deliver domestically and step into a leadership vacuum in Europe.

“Many in Paris place a lot of hope in Merz,” said Camille Grand, a distinguished policy fellow at the European Council on Foreign Relations.

The French, he added, “hope Germany will now move faster and be an active part of these debates: to bolster European defense capabilities and think about how to defend Europe with less or no America.”

Mr. Merz sounded similar themes in his news conference, though he pulled back his critique of Mr. Trump, at least slightly.

“I remain hopeful that we will succeed in maintaining the trans-Atlantic relationship,” he said. He added: “If it were to be destroyed, it would not only be to the detriment of Europe, but also to the detriment of America.”

Central American countries have long taken back their own citizens deported from the United States. But now the Trump administration has called on them to take in people from other countries around the world as well.

The extraordinary measures involved in these deportations — hundreds of migrants whisked away by plane without knowing their destinations and bused to isolated shelters — have shifted attention to Panama and Costa Rica and to how Trump’s immigration crackdown is playing out far beyond U.S. borders.

So far, the number of migrants from elsewhere deported to Central America is still small, and it remains unclear if it will grow. Regional leaders largely say they are actively cooperating with the United States or have downplayed the significance of the deportations. However, analysts warn that these leaders have been backed into a corner with the threat of tariffs and that any increase in deportation flights could eventually push Central America to its limits.

“They’re powerless to do anything,” said Christopher Sabatini, a senior research fellow for Latin America at Chatham House, a research institute in London. “And we saw with President Petro of Colombia the consequences if you resist: sanctions against diplomatic personnel, loss of visa rights, as well as tariffs.”

This month, the Trump administration sent three military planes carrying roughly 300 migrants — mostly from Asia and the Middle East — to Panama. Days later, a flight carrying 135 people, nearly half of them children and including dozens of people from China, Central Asia and Eastern Europe, landed in Costa Rica.

The migrants, who the American authorities say illegally crossed the southern border, are to remain in the custody of the local authorities until they can be returned to their countries or secure asylum somewhere else.

Sending them to other countries removes many of the hurdles that Mr. Trump faced during his first term in trying to curb illegal immigration, according to analysts.

It helps alleviate overcrowding in U.S. detention facilities by removing people from countries like China, Afghanistan and Iran, where a lack of diplomatic relations with the United States makes deportations particularly challenging.

Additionally, the immediate removal of migrants allows the United States to sidestep international legal obligations to offer people who may face life-threatening conditions in their home countries the opportunity to ask for asylum.

Swift deportations also allow the administration to avoid another notably thorny obstacle that Mr. Trump ran into during his first term: Under U.S. law, authorities are not permitted to hold children in detention for more than 20 days, regardless of whether they are with their parents. Of the migrants deported to Central America so far, a large proportion have been families with children.

Publicly, leaders across Central America — clearly worried about the possibility of retribution if they defy the United States — are rejecting the idea that they are being coerced to accept these migrants.

In Panama, officials are characterizing themselves as fully engaged partners on migration. This commitment follows a surge that destabilized the region in recent years, as hundreds of thousands of people crossed into Panama through the Darién Gap, the perilous jungle corridor between Colombia and Panama.

Costa Rica, for its part, has sought to downplay its decision to take in people from distant countries. Officials say it as a one-time request from the U.S. government that involves a negligible number of people. They shrugged off the flight of deportees in a news conference last week, lumping the arrivals in with other migrants who have begun trickling south as the United States and Mexico harden the border.

Still, President Rodrigo Chaves of Costa Rica was frank about his government’s motivation in receiving the migrants: “We are helping the economically powerful brother from the north,” he told a crowd last week, “who, if he puts a tax on the free trade zones, will wreck us.”

Analysts say it is likely that more countries in the region will receive deportees from other countries. Officials in El Salvador and Guatemala have already said that they were willing.

“The biggest problem facing regional governments willing to do Trump’s deportation business is that they must walk the tightrope,” said John Feeley, a former U.S. ambassador to Panama. They have to present themselves as “humanitarian, rule-of-law societies,” he said, even as they stand to look like “cruel henchmen” of the Trump administration.

Costa Rica and Panama have said that along with food, clean water and medical care, the migrants are being given the chance to apply for asylum with the help of United Nations agencies. Local officials have been adamant that they are not sending migrants back to countries where they say they face grave danger.

Panamanian officials have also said that they are not acting under threat.

“There is no quid pro quo, no threats,” Carlos Ruiz-Hernández, Panama’s vice foreign minister, said in an interview. He added that the negotiation with the administration over the Panama Canal — which Mr. Trump has claimed to be under Chinese control — is “compartmentalized” from the agreement to take in migrants deported by the United States.

Accepting the migrant flights is an expansion of an agreement made last summer between Panama and the United States to work together to curb migration, starting at the Darién Gap, said Mr. Ruiz-Hernández.

Panamanian officials have also countered the claim by lawyers there that it is illegal under Panamanian law for the government to detain people for longer than 24 hours without a court order. In the context of immigration, the government legally has “broader powers” to detain people while their migration status is being settled, Mr. Ruiz-Hernández said.

But the government will likely face pushback.

Images like those that appeared in The New York Times, of a migrant from Iran pressed against a hotel window in Panama City, writing “HELP” on the glass, vaulted Panama into the limelight.

Days later, Costa Rica came under similar scrutiny when the country’s ombudsman’s office released a report saying that the migrants deported from the United States had arrived in a state of “visible distress.”

Many did not even know which country they were in, the report said.

Analysts say it is not clear if these Central American nations are getting much in return for their cooperation with the new U.S. deportation approach.

“The truth is, Trump’s not offering them anything,” said Mr. Sabatini, the Latin America expert. “Not development assistance, not international investment.”

Rather, the incentive for cooperation, Mr. Sabatini said, appears to be safeguarding their economies against reprisals by Mr. Trump, who has shown he is willing to mete out high tariffs, even on close allies.

In the climate of fear around Mr. Trump, appeasement and trying to maintain access seems to be Latin America’s response for the time being, Mr. Sabatini added.

S. Fitzgerald Haney, a former U.S. ambassador to Costa Rica, said the Trump administration’s strategy for dealing with leaders in the region was shaping up to be unpredictable.

“At times they’ll be sticks and at times it’ll be carrots,” he said. “But they really want to address security at our southern border.”

David Bolaños contributed reporting from San José, Costa Rica, and Julie Turkewitz from Bogotá, Colombia.

President Trump was barely acknowledged in a meeting between President Volodymyr Zelensky of Ukraine and 13 Western leaders who visited Kyiv in person on Monday to mark the third anniversary of Russia’s full-scale invasion of Ukraine.

Still, he was everywhere. In the subtle rebukes thrown his way. In how European leaders talked about further aid to Ukraine. In how they emphasized the importance of Ukrainian sovereignty, even as Trump officials have been talking about dialing back U.S. support for Kyiv and troop numbers in Europe.

On the invasion’s somber anniversary, European leaders and other Western allies descended on Kyiv to demonstrate their resolute support and pledge more money and military assistance to Ukraine. Ursula von der Leyen, president of the European Commission, the European Union’s executive arm, said strengthening Ukraine’s defenses and energy infrastructure was critically important, and that it was also essential to not back down now.

“The autocrats around the world are watching very carefully,” she said.

The show of solidarity in Ukraine on Monday comes at a head-spinning moment for Europe. For three years, the United States has been a major supporter of Ukraine’s resistance against Russia’s invasion, diplomatically, financially and militarily, pulling the allies together in the leadership role it has played since World War II.

But Mr. Trump is in the process of upending that, or at least threatening to do so.

He shocked European officials last week when he appeared to blame Ukraine’s leaders for Russia’s invasion. He called Mr. Zelensky a “dictator without elections.” And he has been drawing closer to President Vladimir V. Putin of Russia, initiating discussions about ending the war that so far have not involved Ukraine.

On Monday, Mr. Trump said that the fighting could be over “within weeks” and suggested that he could visit Moscow as soon as this spring. He reiterated his demand that Ukraine sign over billions of dollars in mineral rights. In remarks to reporters beside President Emmanuel Macron of France, Mr. Trump refused to call Mr. Putin a dictator, the label he used last week for Mr. Zelensky.

The United States also angered its European allies by voting against a resolution at the U.N. General Assembly that condemned Russian aggression and called for the withdrawal of invading Russian troops from Ukraine. An American resolution simply called for an end to the war.

Because of concerns that Mr. Trump might slash American aid for Ukraine, Mr. Zelensky has been working furiously to shore up European support. And European leaders have been scrambling to come up with a plan to help make up for any change in U.S. engagement.

As the visiting leaders gathered in Kyiv on Monday, European foreign ministers met in Brussels and debated how much aid to send Ukraine in their next support package. Those discussions could yield a package totaling more than 20 billion euros, according to two people familiar with the deliberations, who spoke on the condition of anonymity to discuss internal matters. Kaja Kallas, the E.U.’s top diplomat, said during a news conference on Monday afternoon that the details would be “decided and discussed” on March 6, at a special meeting of European leaders.

The ministers also approved a fresh package of sanctions aimed at Russian energy, trade, transport, infrastructure and financial services. That could displease the White House, as Mr. Trump pivots toward Russia seeking to bring a swift end to the conflict.

“I feel a different sense of urgency, especially after what we all experienced in Munich a couple of weeks ago,” Lars Lokke Rasmussen, foreign minister of Denmark, said Monday on the sidelines of the Brussels meeting, referring to comments by Vice President JD Vance that criticized Europe at a recent security conference. “This is not only about Ukraine. I mean, this is basically about the world order of today.”

The United States has spent about $119 billion on the war in Ukraine, with $67 billion of that going to military spending, according to one frequently used tracker. Europe has dedicated $65 billion to military aid — slightly less — though it has spent $21 billion more than the United States on humanitarian and financial aid.

If the United States were to pull back support from Europe and NATO in a big way, it would be costly and difficult to replace, both in military personnel and in sophisticated military equipment. Even if Europe ordered such hardware now, it would take up to a decade to receive it.

But Europe’s own capabilities were front of mind on Monday, because as much as European leaders are concerned about Ukraine, they are also worried about their own security, which is heavily intertwined.

Mr. Trump has declared that the outcome of the war matters much more to Europe than to America, which is separated from the outcome by what he called a “big, beautiful ocean.”

It is not clear yet whether America will slash military spending in Europe. But European leaders are increasingly worried that the United States could pull out thousands of troops, as Defense Secretary Pete Hegseth recently suggested. That could leave Europe — especially smaller members of the NATO alliance — vulnerable to an aggressive Russia.

Over the last several years, Europeans have been increasing their spending on defense. But they remain far away — in both spending and military capacity — from a level that would allow them to manage without the United States.

European leaders have emphasized their willingness to meet Mr. Trump’s demands that the continent take on more responsibility for its own security.

Friedrich Merz, who is expected to become Germany’s next chancellor, said after his victory in Sunday’s parliamentary elections that it would be an “absolute priority to strengthen Europe as quickly as possible.”

For now, though, European leaders are doing their best to keep the United States at the table, both in regard to military matters of joint importance and when it comes to Ukraine.

Mr. Macron visited Washington on Monday, saying that he will urge Mr. Trump not to “be weak” against Mr. Putin. Mr. Macron has floated the idea of putting European troops on the ground in Ukraine after a settlement to end the fighting — an idea that was initially dismissed by many NATO members and has since gained traction.

Prime Minister Keir Starmer of Britain, who will also visit the White House later in the week, has said he would be willing to commit troops to a peacekeeping initiative — but that it would only work only if the United States acted as a “backstop.”

Mr. Trump has said that there will be no American troops on the ground in Ukraine, but a “backstop” could commit the United States to come to the aid of European peacekeepers if they were attacked by Russia. There is little indication that Mr. Trump would support that.

Mr. Putin, meanwhile, has made it clear that he will not accept the presence of European troops on the ground in Ukraine in any settlement.

The Trump administration’s recent talks with Russian officials about ending the war have raised concerns that Ukraine would be left out of negotiations for any settlement.

On Monday in Kyiv, the visiting European leaders — including two dozen who joined the meeting virtually online — echoed Mr. Zelensky’s oft-repeated talking points: No peace without Ukraine. No peace without a strong security guarantee for Ukraine.

They referred to Mr. Zelensky as “Volodymyr,” and praised him for his courage, with many uttering “Slava Ukraini,” or “glory to Ukraine,” the battle cry of the Ukrainian armed forces.

Prime Minister Justin Trudeau of Canada called Mr. Zelensky a “duly elected democratic leader” — a pointed response to Mr. Trump’s recent insults.

And António Costa, the president of the European Council, said only Ukraine can decide when the conditions are right to start negotiations.

While some leaders offered vague assurances about security guarantees to prevent future aggression or generic promises about more aid, Prime Minister Mette Frederiksen of Denmark was more specific: Membership in NATO for Ukraine was the strongest, easiest and cheapest way forward, she said.

Mr. Zelensky has pushed repeatedly for NATO membership as the one security guarantee his country needs, but the United States and Germany have said that can only happen after the war ends.

Prime Minister Edi Rama of Albania said in a video feed Monday that he felt that the meeting in Kyiv needed to acknowledge the fact that the world order had changed.

“I think that something important has happened, as you all know,” he said, in reference to recent remarks from Washington.

Officials in Brussels also expressed confusion and concern about the trans-Atlantic relationship.

Ms. Kallas, who is going to Washington to meet with administration officials this week, was asked by a reporter if Mr. Trump was operating in a Russian disinformation bubble, as Mr. Zelensky has suggested.

“It’s clear that the Russian narrative is very strongly represented,” she said, carefully choosing her words.

China’s leader said his country and Russia were “true friends who have been through thick and thin together” after a video call with President Vladimir V. Putin on Monday, part of a pointed mutual affirmation of allegiance between Beijing and Moscow as President Trump has turned toward the Kremlin.

The warm words attributed to Xi Jinping in Chinese state media were clearly intended to dampen speculation that the Trump administration, which has pursued a rapid rapprochement with Russia, might succeed in driving a wedge between Beijing and Moscow.

The call came on the anniversary of Russia’s 2022 invasion of Ukraine, after three years in which China extended a lifeline to Russia by helping Mr. Putin weather economic isolation from the West and struggles on the battlefield.

Shortly before the invasion of Ukraine, Mr. Xi and Mr. Putin heralded a “no limits” partnership. Since then, China has sustained Russia’s war machine with oil purchases and exports of dual-use technologies.

Mr. Xi and Mr. Putin also share an ideological opposition to the West. They blame the United States for holding back their global ambitions, and promote a reshaping of the global order to weaken Washington’s dominance.

“History and reality show us that China and Russia are good neighbors who won’t move away, and true friends who have been through thick and thin together, support each other and develop together,” Mr. Xi was quoted as saying by Chinese state media.

Mr. Xi said relations between China and Russia were not “affected by any third party,” in what appeared to be an oblique reference to the United States. And he said the two countries’ foreign policies were there for the “long term.”

The Kremlin issued a similarly cordial statement after the call, describing Mr. Xi and Mr. Putin’s conversation as “warm and friendly.” In a rebuff of the idea that President Trump could divide the two countries, the Kremlin added: “The leaders emphasized that the Russian-Chinese foreign policy link is the most important stabilizing factor in world affairs,” and said the relationship was “not subject to external influence.”

The call was the second between Mr. Xi and Mr. Putin in just over a month, coming less than two weeks after Mr. Trump upended U.S. strategy toward Russia by holding a phone call with Mr. Putin and appearing to side with him over the war in Ukraine. Mr. Trump blamed Ukraine for instigating Russia’s invasion, called President Volodymyr Zelensky of Ukraine a “dictator” and excluded Kyiv from early-stage peace negotiations.

Mr. Trump’s decision to side so favorably with Mr. Putin on the war has fueled speculation that Washington was aiming to split Russia and China, a country that many senior Trump administration officials consider a far more serious threat to U.S. interests.

Some incoming officials in the Trump administration have suggested drawing down American troop levels in Europe, which serve as a bulwark against Russia, so Washington can focus its military efforts on defending against China.

Trump’s special envoy to Ukraine, Keith Kellogg, told a panel earlier this month in Munich that the Trump administration was hoping to “force” Mr. Putin to fray his ties with North Korea, Iran and China.

Analysts, however, have expressed skepticism that China and Russia can be driven apart in what is being called a “reverse Nixon,” a reference to President Nixon’s rapprochement with Beijing in 1972 that was aimed at exploiting the worsening relations between China and the Soviet Union.

Unlike 53 years ago, ties between China and Russia today are at a high, with few prospects for domestic political change in either country.

“There’s strategic and geopolitical alignment for this relationship,” said Sergey Radchenko, a professor at the Johns Hopkins School of Advanced International Studies who specializes in Chinese-Russian relations. “They don’t see eye to eye on everything, but I think they both realize they need each other.”

Mr. Radchenko said Beijing was probably feeling uncomfortable with Mr. Trump’s bid to court Mr. Putin, but that it was unlikely that Mr. Putin would see his interests better served by aligning more closely with the United States over China.

“The idea that Putin can be manipulated as some kind of weapon against China, I think that’s naïve on the part of the Trump administration,” he said.

How much China knew about Russia’s plans in the run-up to Mr. Putin’s invasion of Ukraine three years ago remains unclear.

A Western intelligence report issued shortly after the invasion concluded that senior Chinese officials had told senior Russian officials in early February not to invade Ukraine before the end of the Winter Olympics in Beijing. But the intelligence did not necessarily indicate that direct conversations about an invasion had taken place between Mr. Putin and Mr. Xi.

China has denied any prior knowledge of the invasion, and Beijing didn’t evacuate its embassy or citizens from Ukraine in the days before the start of the war.

“Assertions that China knew about, acquiesced to or tacitly supported this war are purely disinformation,” Qin Gang, the Chinese ambassador to the United States at the time, wrote in a March 2022 article in The Washington Post.

Mr. Xi is set to visit Moscow in May to attend the commemorations of the Soviet victory over Nazi Germany in World War II, according to Russian state media. Mr. Putin has also invited Mr. Trump.

The call on Monday between the Chinese and Russian leaders came as the Kremlin tries to keep its partner nations on its side, while pursuing a warming of relations with the United States, a country Mr. Putin has long derided as an irresponsible global superpower.

“The president last week spoke about his desire to inform a series of government partners about the contacts with the Americans,” the Kremlin’s spokesman, Dmitri S. Peskov, said in a call with reporters on Monday. “In line with those intentions, that process started today.”

The Kremlin added in a statement that Mr. Putin had informed Mr. Xi during their call about “recent Russian-American contacts.” It also said China had “expressed support for the dialogue between Russia and the United States that has begun, as well as readiness to contribute to the search for a peaceful settlement of the Ukrainian conflict.”

Mr. Putin also spoke with the leader of Tajikistan on Monday, and the Russian foreign minister, Sergei V. Lavrov, met with President Recep Tayyip Erdogan of Turkey.

In the comments published by Chinese state media, Mr. Xi said he was “pleased” that Russia had started negotiations with “other parties” to end the “Ukrainian crisis.”

China has yet to describe Russia’s invasion of Ukraine as a “war.”

Amy Chang Chien contributed research.

When the secretary of the treasury, Scott Bessent, traveled to Kyiv this month, he wanted President Volodymyr Zelensky to sign an agreement ceding mineral rights to the United States, delivering a quick win for the Trump administration.

But Mr. Zelensky had an ask of his own: a meeting with President Trump, to finalize a deal he hoped would ensure continued American support. “I hope that in the near future,” he said, “the document will be ready, and we can sign it during a meeting with President Trump.”

Through the crucible of three years of wartime leadership, Mr. Zelensky has mostly played weak hands wisely, like when he popped out of a bunker while his capital was bombed early in the war to film selfie videos rallying his nation and the world to resist. His showmanship also paid off in talks that kept billions of dollars worth of weapons and ammunition coming to his military.

But his approach to the Trump administration has fallen flat with the White House, engendering not empathy but hostility from the American president. His request for a presidential meeting flopped, becoming the latest example of a dramatic personal style that was once integral to his nation’s struggle but now looks more like a monkey wrench in dealing with the Trump administration.

It is hotly debated in Ukraine whether Mr. Zelensky erred in his messaging by responding to insults from Mr. Trump with a few snipes of his own, rather than diplomatically navigating the U.S. president’s attacks. Though Mr. Trump’s claim that Ukraine started the war with Russia was clearly false, Mr. Zelensky infuriated him by publicly correcting the record and claiming the American president was trapped in a “web of disinformation” peddled by the Kremlin.

Was his response a necessary defense of national interests? Or a misstep in dealing with an empowered leader who broaches no criticism and essentially holds Ukraine’s fate in his hands?

“If you are a statesman, you should think first about your country and not your ego,” said Kostiantyn Yelisieiev, a former diplomat and an architect of the playbook used by the former Ukrainian president, Petro Poroshenko, for relations with Mr. Trump.

That approach was characterized by offers of business deals to U.S. companies and responding to Mr. Trump’s criticism of Ukraine with dry, bureaucratic posts on a foreign ministry website.

“It is not a good idea to criticize the leader of any nation, and particularly the leader of a nation doing the utmost to help you,” Mr. Yelisieiev said.

Many Ukrainians have cheered Mr. Zelensky for standing up to Mr. Trump, even if the personal enmity has become an impediment. On Sunday, Mr. Zelensky said he would step down as president if doing so would bring peace to Ukraine, though it was unclear if he was seriously considering that option.

Mr. Zelensky has received advice from alarmed European leaders to avoid escalation, including in a phone call last week with Poland’s president, Andrzej Duda.

“I suggested to President Zelensky to remain committed to the course of calm and constructive cooperation with President Donald Trump,” Mr. Duda wrote on X after the call. Of the American leader he said: “I have no doubt that President Trump is guided by a deep sense of responsibility for global stability and peace.”

Mr. Zelensky’s style has rankled before. In visits to Western capitals to drum up more aid for Ukraine, he lectured leaders to the point of annoyance. The U.K. defense secretary, Ben Wallace, at one point responded that “like it or not, people want to see a bit of gratitude.” And the Ukrainian president has frustrated American military leaders by ignoring their advice on battlefield strategy.

Now, with the future of American military aid and backing in any potential peace talks on the line, it threatens to pose a far bigger problem.

Over the past two weeks, Mr. Zelensky has declined to sign the minerals deal and said he would not accept any outcome of Mr. Trump’s negotiations if Ukraine were not represented. He also has been pursuing diplomatic efforts to shore up European support.

But even backers in Ukraine of this diplomatic strategy say Mr. Zelensky’s showmanship is an issue.

Rather than once laying out Ukraine’s position, Mr. Zelensky reiterated at a security conference in Munich, a news conference in Turkey’s capital and two news conferences in Kyiv that he would reject Mr. Trump’s negotiations if they exclude Ukraine.

The constant public insistence on Ukrainian involvement has irritated Mr. Trump. “He’s been at a meetings for three years, and nothing got done,” Mr. Trump said on Fox News Radio on Friday. “So, I don’t think he’s very important to be at meetings, to be honest with you.”

But the American leader often uses threats and strong-arm tactics as a way of driving things forward — and Mr. Trump may ultimately be fine having Mr. Zelensky involved in the process.

On Sunday, rather than dial down his rhetoric as some European leaders had advised, Mr. Zelensky did not back away from his earlier comment that Mr. Trump is surrounded by Russian “disinformation” about the war.

He pointed to efforts by Mr. Trump to inflate the amount of aid Washington has given to Ukraine. And Mr. Zelensky lingered over Mr. Trump’s belittling assertion that the Ukrainian leader has only a 4 percent approval rating — debunking the claim in what critics say was an unwise war of words.

This is not Ukraine’s first run-in with Mr. Trump. During Mr. Trump’s first term, Ukraine offered deals to buy coal and locomotives from Pennsylvania, handing him a public-relations win in creating jobs in an important electoral swing state. Ukrainian authorities also quietly shut down an investigation of under-the-table payments in Ukraine to Paul Manafort, who had been chairman of Mr. Trump’s 2016 campaign. Mr. Manafort was later convicted of financial crimes in the United States and was pardoned by Mr. Trump.

That approach through Mr. Trump’s first term saw Ukraine get permission to buy Javelin anti-tank missiles — the first lethal military assistance granted to the country — and the imposition of sanctions on Russia’s Nord Stream natural gas pipelines.

In Ukraine today, many say they want a voice in talks that will shape their future — and that Mr. Zelensky’s demand is not just a sign of a stubborn character but a broadly endorsed position in the country. There is little appetite for allowing Mr. Trump’s negotiating team to trade on the army’s achievements in fighting Russia to a near standstill after three years of war — without engagement from Kyiv.

“Ukrainians want peace more than anyone else, but our struggle and the resistance of the Ukrainian military is the only reason why we still exist as a nation, and as a subject of international relations,” said Lt. Pavlo Velychko, who is fighting in northeastern Ukraine. “It was not Zelensky who decided what to want or not to want, but all Ukrainians who stood up to fight.”