When President Donald Trump decided early in his administration to pressure fellow NATO members to spend more on their military budgets, he threatened to pull out of the alliance.

When Iraqi leaders this month said they wanted U.S. troops to leave their country, the president said he would impose "very big sanctions" on Baghdad in response.

And after tensions with Iran recently escalated to the point of potential war, his administration privately threatened large automobile tariffs on European countries if they didn't call out Tehran for alleged violations of the 2015 nuclear deal that Trump has sought to dismantle.

Trump's maximalist approach to diplomacy has become a hallmark of his administration's foreign policy, one that has scored him some short-term victories, been derided as extortion by his detractors and played a central role in an impeachment fight over his actions toward Ukraine that will play out on the floor of the Senate this week.

Although the president has been inconsistent in how he has carried out his worldview, he has made clear that he has no plans to back away from his strong-arm tactics even as they have increasingly antagonized American friend and foe alike, leaving the United States potentially more isolated on the world stage.

Trump heads to Davos, Switzerland, on Monday for an economic forum attended by world leaders and corporate honchos where tensions with his administration will probably be on display. The president is expected to use his address there Tuesday to crow about successful trade deals, a humming U.S. economy and his recent showdown with Iran.

"We are booming. Our country is the hottest country anywhere in the world. There's nothing even close," Trump said Thursday as he confirmed he planned to go to Davos. "Every world leader sees me and they say, 'What have you done? This is the most incredible thing that we've ever seen.' "

Trump's visit to Davos will put him in close quarters with German Chancellor Angela Merkel and executives from European manufacturers days after news that the Trump administration had threatened a 25 percent tariff on European automobiles.

The White House has not announced whether Trump will meet directly with Merkel, although diplomats said they expect the two to talk. Germany has been a central target of Trump's threats on several fronts, as he argues that it does not compensate the United States enough for the military units hosted there and has been allowed to take advantage of economic policies that are unfair to American consumers and companies.

How to deal with Iran is also likely to be a discussion at the forum, particularly if Trump and Merkel meet.

Last week, the Trump administration confounded European officials by threatening to impose the auto tariff if the governments of Britain, France and Germany didn't initiate a mechanism in the Iran nuclear accord that could reimpose an arms embargo and economic sanctions on Tehran. That step, which the three took Tuesday, could eventually unravel the wobbly remains of the Obama-era agreement, though the Europeans are still actively seeking to salvage it. Trump pulled the United States out of the international pact in 2018, but the other signatories to the deal have tried to keep Iran committed to its tenets.

"Extortion," said one European official of the U.S. effort to coerce European foreign policy through tariffs.

That official and others spoke on the condition of anonymity because they were not authorized to discuss the sensitive diplomacy on the record.

The U.S. allies had already planned to initiate the dispute mechanism, said U.S. and European officials, but the threat forced them to re-evaluate their plans for fear of being viewed as bowing to Washington pressure.

"We wanted to do this, but Trump's threat nearly derailed the plans because of how sensitive we are to being perceived as Washington's lap dog," said a European official.

Ultimately, Britain, France and Germany agreed to stay the course and keep the threat a secret. The Washington Post first reported on the administration's backroom dealings, causing embarrassment on both sides of the Atlantic.

"This case demonstrated that the Trump administration has lost the art of diplomacy with allies," said Jeremy Shapiro, director of research at the European Council on Foreign Relations. "The entire nature of a close ally is that you work together to find ways to be in sync without resorting to threats."

The incident also revealed a possible lack of internal coordination on the part of the administration, that is also not uncommon during Trump's presidency. The U.S. ambassador to Germany, Richard Grenell, denied the accuracy of The Post article on Twitter and said he didn't think a tariff threat was ever issued. But David Hale, the undersecretary for political affairs at the State Department, communicated the warning to his European counterparts on Jan. 8, according to European officials familiar with the matter who spoke on the condition of anonymity to address the private discussions.

German defense minister Annegret Kramp-Karrenbauer also confirmed the U.S. threat during a news conference in London last week.

"This expression, or threat, as you will, does exist," she said.

Trump will also be in Davos as the Senate begins debating impeachment charges tied to his actions involving Ukraine. House Democrats allege that Trump tried to leverage a White House meeting and military aid, sought by Ukraine to combat Russian military aggression, to pressure Ukrainian President Volodymyr Zelensky to launch an investigation of former vice president Joe Biden and his son Hunter Biden, as well as a probe of an unfounded theory that Kyiv conspired with Democrats to interfere in the 2016 presidential election.

Trump insists he did nothing wrong and that his concern was with corruption broadly in Ukraine rather than to force investigations that could benefit him in his re-election campaign this year.

And as part of Trump's defense, administration officials have tried to couch his handling of Ukraine policy, which even concerned some Republicans while the aid was withheld over the summer, as fitting perfectly within his strong-arm foreign policy approach as they deny any corrupt intent on Trump's part.

"Elections do have consequences and they should," acting White House chief of staff Mick Mulvaney said during an October news conference. "And your foreign policy is going to change. (President Barack) Obama did it in one way; we're doing it a different way. And there's no problem with that."

In the broader debate over Trump's foreign policy, there have been repeated clashes over both the short- and long-term implications for the United States. While proponents of Trump's coercive style say it has produced results, critics contend that it has hurt American leadership in the world and cost the United States some trust and goodwill among friends.

Trump rankled allies last year when he attempted to extract billions of dollars from them through a formula he coined "cost plus 50 percent," meaning that countries should pay the cost of stationing American troops on their territory plus 50 percent more.

The formula alarmed European officials, most acutely in Germany, where the Pentagon has more than 33,000 troops. After the backlash, defense officials said the formula only pertained to U.S. allies in Asia. Japan and South Korea have not met the cost either, though Seoul has paid more to Washington than it did in the past.

Last year, South Korea agreed to pay $925 million for hosting 28,500 American troops. That was an 8.2 percent increase from the previous year's payment and about half the total costs. South Korean officials preferred a five-year agreement, but the deal covered only one.

On Thursday, Secretary of State Mike Pompeo and Defense Secretary Mark Esper wrote a joint column in the Wall Street Journal proclaiming that "Seoul can and should contribute more to its own national defense."

The article caused unease in Seoul because of its direct implication that South Korea was acting more like a "dependent" than an "ally" - conversations that typically happen behind closed doors rather than in a national newspaper.

Veteran diplomats and analysts argue that Trump shows a dangerous lack of understanding about why U.S. troops are in allied countries - noting the main point is to protect American interests and project power.

"President Trump fails to understand why America has allies in the first place," said Harry Kazianis, an Asia specialist at the Center for the National Interest. "He treats allies more like mafia partners in crime who need to kiss up to America for protection."

Trump says he is using the skills of a real estate magnate to get better deals for Americans whose global generosity he says has been abused. He points to a long-delayed remake of the North American Free Trade Agreement and an interim trade deal with China, both finalized last week, as evidence that tariffs and other punitive tactics are effective.

His supporters also argue that Mexico tightened its immigration enforcement last year because of Trump's threat to impose crushing economic tariffs.

Pompeo said in June that the immigration agreement with Mexico "reflects diplomacy at its finest," while being "a significant win for the American people" and fulfillment of a core Trump promise to secure the U.S.-Mexico border.

But Trump's critics said there is little to no long-term strategic thought to his approach.

"Every day we learn more about the tactics this administration uses to further its goals, and every day we see that they are no more sophisticated than the tactics of gangsters," said Dana Shell Smith, a former ambassador to Qatar who quit in protest of Trump policies.

Fred P. Hochberg, who headed the Export-Import Bank under Obama, said Trump is "always itching for a fight" and prioritizes short-term payoffs to the detriment of the country.

"The United States has prospered by working with others and taking a longer-term view; good relations yield better and more sustainable results for the American people," said Hochberg, author of the new book "Trade is Not a Four Letter Word." "It appears the current thinking is, 'Get what you can now, and don't worry about longer-term consequences,' " Hochberg said.

Some foreign diplomats said that although Trump's style may be more brazen, it isn't unique.

An ambassador from a major U.S. ally described the president's approach as an outgrowth of the New York business world Trump inhabited before becoming a politician.

"Yes, he always wants something out of it," said the diplomat, who requested anonymity to speak freely about interactions with Trump and his aides.

The tactics are unpleasant at times, but by now are not unexpected, the diplomat said. They sometimes work, and with the exception of the potential personal political gain at issue in the Ukraine matter, maybe not all that different from past administrations, the envoy said.

"Washington is extortion writ large," the ambassador said. "I've never seen a more transactional city, and I've been all over the world. Every meeting, somebody wants something from you. Trump always wants something from you, but so does everybody else."

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The Washington Post’s Souad Mekhennet contributed to this report.