By Laurence Norman 

When trans-Atlantic tensions are high, the Munich Security Conference is a stage where they spill into the open.

At the annual event in 2003, arguments over the Iraq war came to a head when an emotional German Foreign Minister Joschka Fischer confronted U.S. Defense Secretary Donald Rumsfeld to insist he wasn't convinced by the American case for removing Saddam Hussein.

In 2015, months after Russia's annexation of Crimea, U.S. Sen. John McCain slammed German Chancellor Angela Merkel for refusing to send defensive arms to Ukraine, saying that supplies of "blankets and meals" weren't enough. "Blankets don't do well against Russian tanks," he said.

Ahead of this year's conference, which starts Friday, strains have emerged between the U.S. and Europe on matters including trade, climate change, Middle East peace and the Iranian nuclear deal.

Relations have been undermined, European diplomats say, by confrontational statements by President Donald Trump, often on Twitter, and the absence of some key U.S. diplomats from a number of unfilled posts.

Old tensions with Washington over European military spending have sharpened under Mr. Trump, who says members of the North Atlantic Treaty Organization should be investing more on defense.

Yet, over a year into the Trump administration, there is tentative confidence among many European diplomats that the core trans-Atlantic security partnership is holding.

"The style of the president has surprised a lot of people but...when I see how NATO discussions are going to reform the command structures...it demonstrates the stability of democratic institutions," said Marc Otte, a former top European Union diplomat who heads Brussels' Egmont Institute think tank.

Unquestionably, there has been discomfort among some European leaders with the new president's policies. After his election, EU foreign policy chief Federica Mogherini called an emergency meeting of foreign ministers to discuss the fallout, a move viewed critically by some capitals.

In May, after a tense trip by Mr. Trump to NATO where he failed to explicitly back the alliance's collective security guarantee, German Chancellor Angela Merkel said the days when Europe could rely on America were "over to a certain extent."

Even U.S. relations with the U.K., traditionally the bridge between Europe and the U.S., have been marred by public spats.

Yet in some areas, trans-Atlantic cooperation has remained the norm. The Europeans have backed Washington's pressure campaign against North Korea, worked with the U.S. against Islamic State and welcomed the Trump administration occasional assertiveness against the Assad regime.

U.S. policy toward Russia has hardened over the past year. Far from seeing a Moscow-Washington rapprochement undercut Europe, EU diplomats found themselves siding last summer with the White House to lobby Congress to tone down new sanctions against Moscow that could have hit European firms.

NATO, once dismissed by Mr. Trump as obsolete, looks steadier. The alliance has absorbed some of Washington's demands and Mr. Trump and his top officials have now spelled out clearly that an attack on one ally remains an attack on all.

Most European countries have responded with increased defense spending, although only half the alliance is expected to meet the goal of spending 2% of economic output on the military by 2024.

That places the Trump administration back into a more traditional dilemma over Europe, said Jan Techau, Director of the Europe Program at the German Marshall Fund of the United States.

"You really want to push very hard that your allies do more on defense and aren't free-riding," he said. "But if you press them too hard, you lose your credibility as a superpower protector and signal to the Russians, the Chinese and everybody else that your security guarantees are vulnerable."

Mr. Trump's nationalist tones have found some allies in Eastern Europe, especially Poland. France's Emmanuel Macron has deployed an engagement strategy with Washington, actively seeking to address U.S. concerns about Iran's regional activities, missile program and some of the weaknesses in the Iran deal.

Some diplomats are wary of assuming things will gradually return to business as usual.

Two issues, in particular, could seriously heighten tensions: Mr. Trump's threats to kill the Iranian nuclear deal in May and any U.S. move to disrupt trade ties.

European officials have repeatedly warned they see the Iran deal as a vital economic and strategic interest. Ending it while diplomatic efforts to raise pressure on Iran are evolving could "create a fallout on a scale we haven't seen before," Mr. Techau said.

The trade threats from Washington are a concern in particular for Berlin, which is worried about threats of punitive tariffs. Germany also sees Washington's attacks on its economic policies as an offensive against the EU, which negotiates trade deals for the bloc, and as a threat to multilateral institutions like the World Trade Organization, which underpin international rules of the game.

"Trump wasn't able to go for the grand bargain he wanted to on Russia but we're only a year into the presidency," said Mark Leonard, director of the European Council on Foreign Relations think tank. "It's pretty clear he is not aligned with the Europeans on many of the key issues."

--Julian E. Barnes contributed to this article.

Write to Laurence Norman at laurence.norman@wsj.com

(END) Dow Jones Newswires

February 15, 2018 12:09 ET (17:09 GMT)