With a staccato burst, a horn sounded in the control room of the H.M.S. Vanguard, sending the crew of the nuclear-armed Royal Navy submarine to battle stations. The voice of the commanding officer crackled over the intercom. “Set condition 1SQ,” he said, ordering its battery of ballistic missiles to be readied for launch.

It was just a drill, conducted last Monday for a visiting V.I.P., Prime Minister Keir Starmer. But Mr. Starmer had reason to pay close attention when he was shown where the submarine’s launch key is stored: The prime minister is the only person in the United Kingdom authorized to order a nuclear strike.

“You’re looking for the ideal conditions?” Mr. Starmer asked softly, as the captain explained how the Vanguard must be maneuvered to the right depth to launch its Trident missiles. Mr. Starmer leaned forward in the captain’s chair, the blue glow from a bank of screens reflected in his eyeglasses.

Later, after he had climbed a 32-foot ladder to the submarine’s deck, Mr. Starmer reflected on its nearly seven-month-long mission. Prowling silently in the depths of the Atlantic Ocean, it is designed to deter a nuclear conflict with Russia (at least one of the four Vanguard-class submarines is always on patrol). At a time when Europe’s capacity to defend itself has come under criticism, not least from President Trump, Mr. Starmer said these mighty boats were an ironclad symbol of Britain’s commitment to NATO.

“Twenty-four hours, 365 days, year after year after year, for 55 years,” Mr. Starmer told me after we had cast off and the Vanguard steamed toward its home port in Scotland. “It has kept the peace for a very long time.”

Back on a tugboat, taking us to shore in the Firth of Clyde, Mr. Starmer sat alone, staring out a window at the gathering clouds. It has been a defining, if sobering, few weeks for the 62-year-old British leader: Swept into power eight months ago on a tide of discontent about the cost of living, he now finds himself fighting to avert a rupture of the post-World War II alliance between Europe and the United States.

“In our heart of hearts, we’ve known this moment was coming from just over three years ago, when Russian tanks rolled across the border” of Ukraine, Mr. Starmer said of Europe’s heightened vulnerability and the strains in the NATO alliance. “We have to treat this as a galvanizing moment and seize the initiative.”

The crisis has transformed Mr. Starmer, turning a methodical, unflashy human rights lawyer and Labour Party politician into something akin to a wartime leader. With debates over welfare reform and the economy largely eclipsed for now by fears about Britain’s national security, Mr. Starmer invoked Winston Churchill and, in a nod to his party, Clement Attlee, the first postwar Labour prime minister, as he described Britain’s singular role in a more fractured West.

“Many people are urging us to choose between the U.S. and Europe,” he said in one of three conversations last week. “Churchill didn’t do it. Attlee didn’t do it. It’d be a big mistake, in my view, to choose now.”

Pausing for a moment, Mr. Starmer added, “I do think that President Trump has a point when he says there needs to be a greater burden borne by European countries for the collective self-defense of Europe.”

The immediate question is whether Britain and Europe will play a meaningful role in Mr. Trump’s negotiations with President Vladimir V. Putin of Russia. To ensure that they do, Mr. Starmer is trying to assemble a multinational military force that he calls a coalition of the willing. The goal, he says, is keep Ukraine’s skies, ports and borders secure after any peace settlement.

“I don’t trust Putin,” Mr. Starmer said. “I’m sure Putin would try to insist that Ukraine should be defenseless after a deal because that gives him what he wants, which is the opportunity to go in again.”

Britain faces hurdles on every front: Russia has rejected the idea of a NATO peacekeeping force. Mr. Trump has yet to offer security guarantees, which Mr. Starmer says are crucial before countries will commit troops. Aside from Britain and France, no other European country has done so, even as Mr. Starmer led the first military planning meeting for the coalition on Thursday.

Senior British military and defense officials said they expected that ultimately, multiple countries would contribute planes, ships or troops to the effort. But regardless of the political and diplomatic uncertainties, Mr. Starmer said he felt he had little choice but to get ahead of the pack.

“If we only move at the pace of the most cautious,” he said, “then we’re going to move very slowly and we’re not going to be in the position we need to be in.”

Behind Mr. Starmer’s whirlwind of diplomacy is an even more elusive goal: persuading Mr. Trump of the value of NATO, the 75-year-old alliance the president disparages as a club of free riders, sheltering under an American security umbrella but failing to pay their fair share.

Unlike President Emmanuel Macron of France or Germany’s incoming chancellor, Friedrich Merz, Mr. Starmer has not called for Europe to chart an independent course from the United States on security. He insists that the “special relationship” is unshakable and that, in any case, British and American forces are deeply intertwined (the United States supplies the Trident missiles on British submarines).

Mr. Starmer has painstakingly cultivated Mr. Trump, phoning him every few days and turning up at the White House last month with a signed invitation from King Charles III for a state visit to Britain. The prime minister said Mr. Trump told him how much he treasured his meetings with Queen Elizabeth II.

The two men could hardly be less alike: Mr. Starmer, disciplined and reserved, with left-wing political roots; Mr. Trump, impulsive and expansive, with habits and instincts that shade into the regal. Yet they seem to have established a rapport. Mr. Trump occasionally calls him on his cellphone, one of Mr. Starmer’s aides said, to discuss favorite topics like his golf resorts in Scotland.

“On a person-to-person basis, I think we have a good relationship,” Mr. Starmer said of Mr. Trump, whom he first met over dinner in Trump Tower last fall. “I like and respect him. I understand what he’s trying to achieve.”

As for Mr. Trump’s actions — from imposing a 25 percent tariff on British steel to berating President Volodymyr Zelensky of Ukraine — Mr. Starmer said he recognized that the president had generated “quite a degree of disorientation.” The right response, he said, was not to get provoked by it.

“On the day in which the Oval Office meeting between President Trump and President Zelensky didn’t go particularly well, we were under pressure to come out very critically with, you know, flowery adjectives to describe how others felt,” Mr. Starmer recalled. “I took the view that it was better to pick up the phone and talk to both sides to try and get them back on the same page.”

Mr. Starmer dispatched his national security adviser, Jonathan Powell, to Kyiv, the Ukrainian capital, to coach Mr. Zelensky on how he could mend fences with Mr. Trump. In multiple sessions, two senior British officials said, they crafted language to mitigate Mr. Zelensky’s anxieties about a cease-fire in which the Russians would keep shooting.

Mr. Starmer then phoned Mr. Trump to relay the progress in Kyiv and lay the groundwork for a call between him and Mr. Zelensky. When the presidents spoke again, Mr. Zelensky threw his support behind Mr. Trump’s peacemaking effort.

In offering himself as a bridge, Mr. Starmer is trying to reclaim a role that Britain played for decades before it voted to leave the European Union in 2016. It showed, he said, that after a period in which Britain had been “disinterested” and “absent” from the world stage, “we’re back, if you like.”

But there are limits to Britain’s role in a post-Brexit world: The E.U. said it would exclude British weapons manufacturers from a defense fund worth 150 billion euros ($162 billion), unless Britain signs a security partnership agreement with Brussels. Britain, analysts say, will find it harder to act as a bridge if Mr. Trump spares it from more sweeping tariffs that he has vowed to impose on the European Union.

For now, Mr. Starmer’s statesmanship has buoyed his poll ratings and won him praise across the political spectrum. After a fitful start, in which he was dogged by a torpid economy, Mr. Starmer said the crisis “had injected an urgency” into his government.

How long that will last is anyone’s guess. Britain’s economy continues to sputter and Mr. Starmer has faced a backlash over decisions like cutting payments to help retirees with winter heating costs. The benefits of being a statesman, analysts say, can be evanescent if domestic woes keep piling up.

Even the fire at an electrical substation in London on Friday, which shut down Heathrow Airport and threw travel plans for tens of thousands into chaos, is a reminder of how events can temporarily swamp a government’s agenda.

Painful trade-offs loom, further down the road. Mr. Starmer has pledged to increase military spending to 2.5 percent of Britain’s gross domestic product by 2027, financed with a cut to overseas development aid. It is not clear how Britain will pay for a promised further increase to 3 percent of G.D.P. within a decade.

“We’ve all enjoyed the peace dividend,” Mr. Starmer said, noting that Europe is moving into a darker era. “I don’t want to veer into scaremongering,” he said, but added, “We need to think about defense and security in a more immediate way.”

Three days after the submarine visit, Mr. Starmer took part in a keel-laying ceremony for a new fleet of ballistic missile submarines, being built at a shipyard in Barrow-in-Furness, in northwest England. Four Dreadnought-class vessels, each almost the length of St. Paul’s Cathedral, are scheduled to go into service in the early 2030s, at an estimated cost of 41 billion pounds ($53 billion).

Standing in the cavernous factory, with the aft section of a submarine towering above him, Mr. Starmer expressed pride in this statement of British might. But it was also a reminder of the stretched state of its military.

The Vanguard-class submarines being replaced by the Dreadnoughts are nearly 30 years old — “pretty old kit,” in Mr. Starmer’s words — which necessitates prolonged maintenance periods. That has extended the patrols for the other vessels in the fleet and put acute pressure on their roughly 130-person crews.

The strain was on display during Mr. Starmer’s visit to the Vanguard, which set a Royal Navy record for longest patrol. Sailors said the food, excellent at first, deteriorated as the submarine’s provisions dwindled. Four were returning to spouses who’d had babies while they were away. Others lost family members, only learning the news from the captain on the eve of their return.

“It is with huge respect to the team,” that they survived seven months at sea, Mr. Starmer said after stepping gingerly off the submarine’s weathered deck. “But we shouldn’t be celebrating it.”

“This has doubled my resolve to ensure we go further and faster in our capabilities,” he said, “to make sure they are not put in that position again.”

When the first buses of newly freed migrants arrived this month in Panama City from a detention camp at the edge of a jungle, three people were visibly ill. One needed H.I.V. treatment, a lawyer said, another had run out of insulin and a third was suffering from seizures.

Confusion, chaos and fear reigned. “What am I going to do?” one migrant wondered aloud. “Where am I going to go?”

These are questions being asked by dozens of migrants deported to Panama last month by the Trump administration, part of the president’s sweeping efforts to expel millions of people from the United States.

At first, Panamanian officials had locked the group of about 300 people in a hotel. Then, those who did not accept repatriation to their home countries were sent to a guarded camp at the edge of a jungle. Finally, after a lawsuit and an outcry from human rights groups, the Panamanian authorities released the deportees, busing them back to Panama City.

Now, the remaining migrants — from Iran, Afghanistan, Cameroon, Ethiopia, Uzbekistan and elsewhere — are free but stranded in a country that doesn’t want them, many sleeping in a school gymnasium made available by an aid group, with no real sense of what to do next.

Interviews with 25 of the deportees offered a revealing look at who is being pushed out of the United States by the Trump administration, and what happens once they arrive in Central America.

The region has emerged as a key cog in the deportation machinery President Trump is trying to kick into high gear.

But Washington’s decision to send migrants from around the world to Central America has also raised legal questions, tested governments seemingly unprepared to receive migrants and left people marooned in nations where they have no support networks or long-term legal status.

Most of the migrants in Panama said that when they arrived in the United States they told officials they were fearful of returning to their countries, but were never given an opportunity to formally ask for asylum.

A spokeswoman for the Department of Homeland Security, Tricia McLaughlin, said in an email that the migrants had been “properly removed” from the United States. She added that “not a single one of these aliens asserted fear of returning to their home country at any point during processing or custody.”

“The U.S. government coordinated for the welfare of these aliens to also be managed by humanitarian groups in Panama,” she said.

Since taking office, Mr. Trump has sent hundreds of migrants from around the world to Panama, Costa Rica and El Salvador, though it is unclear if the U.S. government plans to continue doing so.

“Whether there will be more planes from the United States or not, I honestly don’t know,” Panama’s president, Raúl Mulino, said this month. “I’m not very inclined to do it, because they leave us with the problem.’’

Those now stranded in Panama include Hedayatullah Zazai, 34, a man who said he had served as an officer in the Afghan Army, working alongside U.S. Special Forces and American consultants. After the Taliban took over, he fled to Pakistan, he said, then Iran, then flew to Brazil and trekked through South and Central America to get to the U.S. border.

The deportees also include Iranian Christians who said they were under threat at home, and several Afghan women from the Hazara ethnic minority who say they face persecution under the Taliban.

Another deportee is Simegnat, 37, an Amhara woman traveling alone from Ethiopia who said she had been targeted by her government because her ethnicity led the authorities to suspect her of working with a rebel group. She said she fled after her home was set on fire, her father and brother were killed and the police told her she would be next.

“I was not a person who wanted to flee my country,” she said. “I owned a restaurant and I had a good life.”

“We are humans, but we have nowhere to live,” she said of the Amhara people.

She and several of the migrants, fearing for the safety of relatives back home, asked not to be identified by their full names.

Most of the migrants described crossing the Mexico-U. S. border early this year, being held for about two weeks in detention, then shackled by U.S. officials and put on a plane to an unknown destination. Some said they had been told they were headed from California to Texas; most said they were never given an opportunity to formally ask for asylum.

One 19-year-old woman from Afghanistan said U.S. officials had permitted her parents and five younger siblings to cross the border into the United States. As the only sibling over 18, she was separated from them, detained and flown to Panama, she said.

Some said they owed hundreds or thousands of dollars to people who helped them fund their journeys.

“If I go back to Ethiopia without their money,” Simegnat said, “they would kill me.”

Panama has given the deportees 30-day permits that allow them to stay in the country for the time being and has given them the option of extending their stay to 90 days.

While Panama has an asylum program, migrants have received mixed messages about the likelihood of receiving long-term legal protections in the country, they said.

Another option is for individuals to find another country that will take them. But that would require a case-by-case legal effort, said Silvia Serna, a lawyer who is part of the team that filed a lawsuit that called Panama’s detention of the migrants at the hotel and border camp was illegal.

Ms. Serna said she had been interviewing the migrants to see what assistance her team could offer, but cautioned that it might be very hard for people to find welcoming countries.

In interviews, three of the Iranian deportees said they planned to turn around and head back to the United States and were already negotiating with a smuggler. A fourth had already left for the U.S. border.

One is Negin, 24, who identified herself as a gay woman from Iran, where openly gay people face government persecution. “At least if I’m lingering idly,” she said, “I’ll be inside an American detention camp and on American soil.”

The smuggler quoted one woman a price of $5,000 to get her across the U.S.-Mexico border in Tijuana, and $8,000 to secure her a visa and put her on a plane to Canada.

For now, most of the group is staying at a school gymnasium-turned-shelter outside Panama City run by two Christian charities. The migrants sleep on thin mattresses and eat meals from plastic foam containers. A group of them went door to door at various embassies this week asking for help, but said they had been rejected at every one.

Elías Cornejo, who works with one of the aid groups, Fe y Alegría, was unsparing in his criticism of the new U.S. administration.

“We think that the policies of the Trump administration are part of a machine that grinds the migrant like meat,” he said. “And that obviously is a serious problem of inhumanity.”

A smaller group of deportees, mostly families with children, has been staying at a hotel in Panama City paid for by UNICEF. Among them is a married couple, Mohammad and Mona who are Christian converts from Iran. One night, as their 8-year-old son broke down, both parents leaned over him, stroking his face.

“He doesn’t go to school and life has become repetitive for him,” Mohammad said.

The couple had considered re-entering the United States illegally, they said, and eventually decided they could not put their child through more suffering. They are holding out hope that a lawyer on Ms. Serna’s team can persuade the Trump administration to grant them entry as persecuted Christians.

If that doesn’t work, Mohammad said, he was considering staying in Panama and was already looking for work.

Not far from the hotel recently, Artemis Ghasemzadeh, 27, another Iranian Christian, entered a white-walled church and knelt in a pew. Ms. Ghasemzadeh became something of a leader of the group after she posted a video online from detention at the Panama City hotel, pleading with the world for help.

She said that a priest had offered the migrants group housing north of Panama City, where they would be welcome to stay as long as they were in the country. The houses have kitchens, and they would have no curfew, she added. She was mulling over the offer.

“I don’t know what will happen next,” Ms. Ghasemzadeh said. “I don’t know my next step. At the moment, we are in God’s hands.”

Reporting was contributed by Alex E. Hernández from Panama City, Ruhullah Khapalwak from Vancouver, British Columbia, and a New York Times reporter from Addis Ababa, Ethiopia.

The mayor of Istanbul was jailed pending his trial on corruption charges and removed from office on Sunday, hobbling a potential contender in Turkey’s next presidential election and the top rival of President Recep Tayyip Erdogan.

The mayor, Ekrem Imamoglu, was arrested at his home on Wednesday, four days before he was set to be named the presidential candidate of Turkey’s political opposition. He has denied the accusations against him, which Mr. Erdogan’s opponents have called a ploy to hinder a popular politician’s presidential bid.

The court ordered that Mr. Imamoglu be jailed on accusations of corruption pending a trial. State prosecutors have accused him of leading a criminal organization and overseeing bribery, bid rigging and other financial misdeeds at City Hall.

Prosecutors also accused him of supporting terrorism through his political coordination with a pro-Kurdish group during local elections last year. The court chose not to order his detention on those charges, but said he is under grave suspicion.

On Sunday afternoon, the Interior Ministry removed Mr. Imamoglu from office.

Detention alone may not prevent him from running for president, but he faces other roadblocks. The day before his arrest, his alma mater, Istanbul University, voided his diploma, citing improper procedures in his transfer to the school in 1990. Turkey’s Constitution stipulates that the president must have completed higher education. The mayor said before he was detained that he would contest the ruling.

Mr. Imamoglu, who has been elected mayor three times since 2019, faces a slew of other court cases as well, including some that could temporarily bar him from politics.

In a post on X, the mayor called on Turks to stand together against “this black stain on our democracy.”

Of his detention, he said: “I stand tall. I will never bow.”

Critics of Mr. Erdogan, who has dominated Turkish politics for more than two decades, have long accused him of using state power to undermine his rivals. But, they say, arresting a presidential contender to undermine him in the race before it begins represents a new level of authoritarianism. His arrest has prompted the largest street protests in Turkey in more than a decade.

Some European leaders have criticized the mayor’s arrest, and they have called on the Turkish government to uphold the rule of law. Senior U.S. officials have said little.

Steve Witkoff, President Trump’s Middle East envoy, made no mention of the mayor’s detention in an interview with the former Fox News anchor Tucker Carlson that was posted on X on Saturday. But he said that Mr. Trump had recently spoken with Mr. Erdogan. The call was not made public by the White House at the time.

“There is just a lot of good, positive news coming out of Turkey right now as a result of that conversation,” Mr. Witkoff said, without providing further details.

The court also jailed dozens of other people, according to the state-run Anadolu news agency, including several of Mr. Imamoglu’s associates. At least four of his close aides were jailed on corruption charges and two others for supporting terrorism, the state-run news media reported.

Two Istanbul district mayors were also jailed and removed from their posts, bringing the total removed by the government in recent months to five. All of the ousted mayors are from Mr. Imamoglu’s party, which won 26 of Istanbul’s 39 districts in local elections last year.

Despite Mr. Imamoglu’s detention, Turkey’s main opposition party, the Republican People’s Party, or C.H.P., went ahead with a primary on Sunday to designate him its presidential candidate. Party members voted around the country, and the party called on nonparty members to turn out as well to cast symbolic ballots to support the jailed mayor.

At one polling station in a neighborhood that Mr. Erdogan’s party won in local elections last year, about 20 people stood in line to cast symbolic votes for Mr. Imamoglu.

“I am here for our freedom. I am here for my children,” said Hilal Dukmeler, a 26-year-old nurse. “If we keep silent on this one, our republic will not survive.”

She said Mr. Erdogan’s party had failed to offer services and considered Mr. Imamoglu a threat.

“This is show of power of the government,” she said. “They realized that Imamoglu has the power too and they tried to beat him.”

Mr. Erdogan’s current presidential term, his second, expires in 2028. While the Constitution limits presidents to two full terms, he could legally run again if Parliament called for early elections, cutting short his second mandate.

Many people in Turkey expect that to happen. If it does, it is possible that Mr. Imamoglu, 54, could be barred from the race. Mr. Erdogan, 71, has not said whether he would run, but he has no clear successor and many people in Turkey expect him to seek another term.

The removal of Mr. Imamoglu could disrupt the administration of Turkey’s largest city. The city government employs more than 100,000 people and oversees a number of companies that build housing, run public transportation and carry out infrastructure projects.

Large protests against Mr. Imamoglu’s detention have broken out nightly in cities across Turkey and more were planned for Sunday, despite the government’s efforts to stop them. Public demonstrations have been banned in the country’s three largest cities, social medial access has been restricted and major transit hubs have been closed to hamper the ability of protesters to gather in public squares.

On Saturday, the Interior Ministry said that 343 people had been arrested while protesting, and the office of Istanbul’s governor, who is appointed by Mr. Erdogan, said travelers “likely to participate in unlawful protests” would be prevented from entering the city.

Gulsin Harman contributed reporting.

A foul cell in a Moscow detention center was about the last place an American businessman named Michael Calvey expected to find himself after spending 25 years building a flourishing venture capital firm in Russia that transformed some tech startups into global brands.

First, beefy agents from the F.S.B., the federal security service, ransacked his apartment before dawn. Hours later he was confined to a holding cell with two other inmates and a filthy hole in the floor for a toilet.

“The cell is stuffy and hot, an oppressive stench hanging in the air as if from accumulated decades of human sweat mixed with the indescribable horrors emanating from the toilet hole area,” Mr. Calvey wrote in a new book out this week called “Odyssey Moscow.” It details his extended ordeal through the Russian court system in a fabricated fraud case initiated in 2019: “In the course of a few surreal, terrifying hours I have morphed from one of the most successful Western businessmen in Russia into a prisoner of the state.”

With President Trump lauding the possibility of “major economic development transactions” between the United States and Russia as he seeks improved relations with Moscow, Mr. Calvey’s fate stands as a cautionary tale about the significant personal and professional risks involved in doing business in Russia, particularly given the arbitrary nature of its courts.

Perhaps no Western businessman promoted foreign investment in Russia more than Mr. Calvey, 57, who helped to forge internet titans from tech startups like Yandex — a version of Google, Amazon and Uber rolled into one — or Tinkoff Credit Systems, one of the world’s biggest digital banks. The firm he founded, Baring Vostok Capital Partners, earned colossal returns.

Then Baring Vostok got mired in a nasty commercial dispute with two dubious Russian partners who were stripping assets out of a bank in a troubled merger. Once, Mr. Calvey’s empty Moscow apartment mysteriously caught fire hours before a dinner involving tense negotiations.

After his firm filed a case with a London arbitration court, the partners convinced Department K of the F.S.B., responsible for internal financial crimes, that the American and several partners had perpetuated a massive fraud as part of a dastardly foreign plot to undermine Russia’s financial sector.

The agents pounced in February 2019, and although no evidence of wrongdoing ever emerged in court, Mr. Calvey and several partners spent years in jail or under house arrest.

“Once the F.S.B. gets involved in a case, they’re like a car with six gears going forward and none in reverse,” Mr. Calvey said in an interview in Switzerland, his home since finally being allowed to leave Russia in 2022. Lanky and trim, he retains a boyish air despite his gray hair. “They will never back up or lose face.”

His arrest stunned Western investors. “Everyone I knew was incredulous, angry and shocked,” said Bernie Sucher, an American banker with extended experience in Russia. “It was viewed as a direct assault on the very idea of long-term investment in the Russian economy.”

Unusually, dozens of influential Russians defended Mr. Calvey. They included Kirill Dmitriev, the head of Russia’s sovereign wealth fund and now a key negotiator for ending the Ukraine war; German Gref, the chief executive of Russia’s largest bank; and Alexei Kudrin, a previous finance minister. The U.S. Embassy in Moscow also objected strenuously to his arrest.

Mr. Calvey thought such interventions, combined with the blow to investor confidence, would get the case dropped. But nothing outweighed the F.S.B.

President Vladimir V. Putin did summon top Kremlin officials, ordering them to get the American businessman out of prison, but also to find something illegal that Mr. Calvey had done, he said he later learned. At a tense time in U.S.-Russia relations, the Kremlin could not admit to arresting a prominent American businessman on false pretenses, he said.

Released from prison after two months, Mr. Calvey was confined to his apartment with an electronic monitoring device strapped around his ankle for two years, and spent a third under court-ordered supervision with an 8 p.m. curfew. When he developed a cancerous tumor in one leg, the court refused to allow him to remove the device, so doctors operated without benefit of an M.R.I.

The Russian Foreign Ministry and the Russian Embassy in Washington did not respond to requests for comment about Mr. Calvey’s account. At the time of his conviction, Dmitry Peskov, the presidential spokesman, quoted Mr. Putin as saying that the government could not interfere in the courts.

When first arrested, Mr. Calvey was jailed in Matrosskaya Tishina prison, near downtown Moscow. It is sometimes called “Kremlin Central” because so many inmates face charges in high-profile corruption cases pushed by the Kremlin. There were no violent criminals, but nobody is ever acquitted, either, Mr. Calvey wrote.

His cellmates greeted him with a nonalcoholic toast: “Novoselye,” or welcome. One was a former deputy minister of culture. Another was an army general. A younger one was a computer hacker, and three were construction moguls. Trust nobody, one of them confided.

Their cell, 13 feet by 16 feet, was tidy and somewhat comfortable, with a television and a separate toilet. The men shared everything equally from cleaning chores to food supplies from outside. He dedicated his book to the men of Cell 604, and tears up when he talks about them. The book will be released Thursday in Britain and in early April in the United States.

Throughout his detention, Mr. Calvey endeavored to avoid his jailers seeing him disturbed. His reading list included Kafka as an apt reflection of his fate.

When one prosecutor summarized the case, for example, she admitted that not a single witness testified to a crime being committed, then added, “That just proves what a well-organized criminal group we are dealing with.” The entire courtroom laughed aloud, Mr. Calvey said.

The trial underscored F.S.B. control over the courts, with the closing statements repeating the opening accusations almost exactly, Mr. Calvey said. All the witness testimony might never have happened. “Russian people are of course the main victims of its courts,” he wrote.

In August 2021, Mr. Calvey was convicted of the misappropriation of funds and given a five-year suspended sentence. The conviction on false charges grated, he said, a stain on all his work for Russia.

His Russia saga started in 1991, when just two years out of the University of Oklahoma, Mr. Calvey went to work for his former Wall Street boss at the European Bank for Reconstruction and Development. It was established to help the former Soviet bloc transition to a market economy.

He worked on financing energy sector projects. Considered young for the magnitude of the deals, he tried to camouflage his age by adopting a serious demeanor at work, said Charlie Ryan, his first Moscow roommate.

“Life for an expat in 1990s Moscow was equal parts bizarre and marvelous,” Mr. Calvey wrote. Pizza Hut was considered a high-end restaurant to impress a date. Kilos of inexpensive caviar proved a substitute for breakfast cereal.

Mr. Calvey established Baring Vostok to build businesses catering to the new middle class. He married a Russian woman named Julia, with whom he had two sons and a daughter, now all young adults.

He existed within an elite business bubble, surrounded by people eager to integrate Russia into the global economy. At the time of his trial, Baring Vostok said that overall, it had invested more than $2.8 billion in 80 companies across the region, making it the biggest such Western player.

He learned Russian through countless hours he spent with young, ambitious entrepreneurs. “It was hard to spend time with them and not feel like Russia was a much, much better place than at the time of their grandparent’s generation,” he said.

When prominent businessmen got arrested, Mr. Calvey attributed it to their meddling in politics. He considered his Russian associates overly gloomy about the direction of their country.

He ignored repeated red flags that Mr. Putin, a former K.G.B. agent, had handed control over every major institution to the siloviki, a Russian term incorporating all security agencies. Not even the illegal annexation of Crimea in 2014 deterred Mr. Calvey.

“What I didn’t really appreciate, and only realized with my arrest, was the depth of the control and influence of the ruling caste of Russia, which is F.S.B. and the other siloviki,” he said.

Mr. Calvey’s businesses thrived even while he was imprisoned, and he pulled the plug only after Russia invaded Ukraine in 2022. The hasty disinvestment cost his company billions of dollars, he said.

He is done with Russia. Although under Russian law his conviction was nullified after his five-year probation period ended a year ago, last week a Moscow court changed the probationary sentence given to a French defendant in the case to a prison term in absentia.

Mr. Calvey expects some American businesses to return, although he considers Russia too risky for long-term investments. A peace deal might prompt him to invest in Ukraine, however. He is fostering internet startups elsewhere, employing young tech talent that fled Russia.

The simmering geopolitical differences between Moscow and Washington mean that any businessman can become a chessboard pawn, he said, adding: “You may hope that you’re not going to get stepped on the head, but ultimately it could happen at any time.”