Talks between the United States and Iran, which President Trump said on Monday would begin on Saturday in Oman, face considerable problems of substance and well-earned mistrust.

But time is short for what is likely to be a complicated negotiation.

“We’re at a fork in the road, heading toward a crisis,” said Sanam Vakil, the director of the Middle East and North Africa Program at Chatham House.

While Mr. Trump has recently threatened Iran with “bombing the likes of which they have never seen before,” he has also made it clear that he prefers a diplomatic deal. That reassurance — made in the Oval Office sitting next to Prime Minister Benjamin Netanyahu of Israel, who has pressed for military action — will be welcomed widely in the Arab world.

Even if the target is the Islamic Republic of Iran, with all of its ambitions for regional hegemony, Arab countries from Egypt through the Gulf fear the economic and social consequences of an American and Israeli war, especially as the killing in Gaza continues.

But Mr. Trump’s public demands — that Iran stop nuclear enrichment, hand over its large supply of enriched uranium and destroy its existing nuclear facilities — will almost surely be rejected by Ayatollah Ali Khamenei, Iran’s supreme leader, as an unacceptable humiliation and surrender. How far both sides are willing to compromise is unclear, but Mr. Trump is well known for making ultimate demands at the start and then searching for a deal.

This weekend’s talks are expected to be at a high level and include Iran’s foreign minister, Abbas Araghchi, and reportedly Steve Witkoff, Mr. Trump’s special envoy for the Middle East, Russia and much else. While the two sides disagree on whether these initial talks will be “direct,” as Mr. Trump said, or “indirect” through intermediaries, as Iran said, it will not matter very much, given the importance of the two men.

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The European Union is responding to President Trump’s sweeping trade war with a handshake and a punch: It is promising the administration potential wins while also preparing its own retaliatory tariffs on American products starting next week.

The questions are whether the enticements are enough, and whether a show of strength could backfire.

“Europe can hurt America, and retaliating seems like a good strategy if you believe that Trump cares about the political fallout from economic pain here at home,” said Michael Strain, director of economic policy studies at the conservative think tank American Enterprise Institute, in Washington. “The worry is that he doesn’t care.”

Mr. Trump has threatened to impose huge additional tariffs on Chinese goods to punish the nation for retaliating against his previous tariffs, and his team appears to be giving some nations that did not retaliate and have close economic ties to the U.S. — notably Japan — priority in negotiations.

At the same time, Mr. Trump has yet to grab the carrots that Europe has dangled in front of him. Ursula von der Leyen, the president of the European Commission, offered on Monday to drop tariffs on imported American cars and other industrial products to zero if the U.S. does the same, a “zero-for-zero” strategy. Asked about that possibility, Mr. Trump said “it’s not” enough to make him back down.

Instead, the administration appears to be standing by its spate of recently-announced tariffs, at least for now. The Trump administration has announced 20 percent across-the-board levies on the E.U., in addition to even higher ones on steel, aluminum and cars.

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Hanging from a ladder attached to the hull of a giant, bobbing tanker, Capt. Efraín Hallax began his climb up to the crew awaiting him atop the Athina, a ship anchored in Panama Bay and third in line to transit the Panama Canal.

The daunting ascent up the shifting ship was nothing new for Captain Hallax, 73. He has been a ship’s pilot in the Panama Canal for over 40 years, responsible for guiding vessels through the trade-critical corridor that connects the Pacific Ocean with the Caribbean Sea.

On this night in February, Captain Hallax reported for work a half-hour before midnight — and just a few hours after President Trump had canceled a call with President José Raúl Mulino of Panama to continue their negotiations over the future of the canal, which the U.S. president wants returned to American control.

The overnight crossing was nothing out of the ordinary for the captain. With about 3 percent of the world’s maritime trade carried through its locks, the Panama Canal is a 24-hour operation.

“Business as usual,” said Captain Hallax. “Always business as usual in the Panama Canal, no matter Christmas, no matter rain, no matter fog, no matter nothing, no matter Trump.”

Xi Jinping wants you to know that he will not be cowed.

Confronted with the latest threat from President Trump of an additional 50 percent tariff on Chinese goods unless Beijing reverses its retaliatory levies on U.S. imports, China’s top leader has remained defiant. His Ministry of Commerce on Tuesday accused the United States of “blackmail” and declared that Beijing would “fight to the end.”

But behind the bravado is a more complicated set of realities for Mr. Xi that makes it politically and economically untenable to offer concessions to the country’s single largest trading partner and chief rival for global influence. With Mr. Trump also refusing to back down, a devastating trade war between the two largest economies may be inevitable — a showdown with painful consequences that will be felt across the world.

The dilemma for Mr. Xi is that looking weak is not an option, but hitting back risks further escalation. The Chinese leader has cast himself as a national savior who is rejuvenating his country’s greatness. As a result, Beijing has less flexibility to back down from a fight with Washington, as other U.S. trading partners like Vietnam have tried, because it could undercut Mr. Xi’s legitimacy, analysts say.

“Beijing’s response to date has emphasized three things: resolve, resilience, and retaliation,” said Julian Gewirtz, a former senior China policy official at the White House and State Department under President Biden who is now writing a book on U.S.-China relations.

“Xi has built up an image of himself as a defiant strongman helming a powerful country, and China’s official messaging is conveying that they are determined to stand up to U.S. pressure even at high costs,” he said.

A major Western leader announces an unorthodox economic policy, panicking the financial markets, driving down the country’s currency and fueling a blizzard of warnings about the dire long-term consequences.

President Trump did all this with his across-the-board tariffs, announced last week, but before him, there was Liz Truss, Britain’s former prime minister, with her rollout of sweeping tax cuts over 44 turbulent days in the fall of 2022.

The parallels between Mr. Trump and Ms. Truss are striking, but for one crucial difference: She was forced to rescind the tax cuts within days, and forced out of office by her own Conservative Party in little more than six weeks, the shortest tenure for a prime minister in British history.

To some analysts, that difference is a tribute to the flexibility of Britain’s parliamentary government and a salutary distinction between Britain and the United States. So far, Mr. Trump has vowed to stick with his tariffs, no matter the carnage they wreak in the markets or whether they trigger a recession, and there seems to be little anybody can do to force him to change course.

“Truss could really only damage the United Kingdom,” said Jonathan Portes, a professor of economics and public policy at Kings College London. “Ultimately, U.K. institutions, in particular Parliament and the media, were enough to ensure that the system worked.”

“Whether that is the case in the U.S. remains to be seen,” he added. “If it isn’t, the whole world will pay the price.”

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President Volodymyr Zelensky of Ukraine said on Tuesday that two Chinese citizens fighting alongside Russian forces had been taken as prisoners of war.

Mr. Zelensky said that the two Chinese citizens had been captured while fighting in the Donetsk region of eastern Ukraine. He added that Kyiv had information that “there are many more such Chinese citizens in the enemy’s units,” without providing evidence. The claims could not be independently verified, and there was no immediate comment from the Chinese or Russian governments.

The announcement came at a fraught moment for Ukraine as the Trump administration has seemingly drawn closer to Russia while trying to act as a mediator in cease-fire talks.

Mr. Zelensky on Tuesday did not suggest that the Chinese fighters had been sent by China’s military, but he pointed to their presence as further evidence that President Vladimir V. Putin of Russia was not truly interested in peace.

“Russia’s involvement of China, along with other countries, whether directly or indirectly, in this war in Europe is a clear signal that Putin intends to do anything but end the war. He is looking for ways to continue fighting,” he wrote on social media.

Mr. Zelensky said that he had instructed his foreign minister to “immediately contact Beijing and clarify how China intends to respond to this.” The foreign minister, Andrii Sybiha, confirmed that he had summoned the Chinese chargé d’affaires to demand an explanation.

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