BRUSSELS -- When Donald Trump walks into a NATO summit Wednesday, international politics are bound to become intensely personal -- again.
Since moving into the White House almost 18 months ago, the U.S. president has been as abrasive with some of America's most trusted allies as he has been warm toward traditional adversaries like North Korea and Russia.
The unpredictable Trump's affinity for the leaders of NATO's 28 other members carries the potential to further blur the lines between who is a friend and who is a foe.
The iconic visual from the military alliance's 2017 summit was Trump shoving aside the prime minister of new member Montenegro, Dusko Markovic, to get in front of the pack for a group photo. The billionaire went on to publicly scold his fellow NATO leaders over defence spending.
Last years' visit to Brussels also produced the famous mano a mano handshake between Trump and French President Emmanuel Macron. The two men, who were meeting for the first time, locked hands with grips so intense their knuckles started turning white.
Divisive issues have generated other white-knuckle moments in the year since. Trump still is angry the allies don't beef up their individual military budgets. The Europeans abhor Trump's decision to impose steel and aluminum tariffs on a part of the world that is supposed to be the United States' partner.
The dynamics between Trump and heads of government with their own predilections could create some volatile chemistry at the two-day summit.
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ANGELA MERKEL
Merkel and Trump are a study in contrast. The German chancellor is as careful with her policies and public statements as the U.S. leader is brash and unpredictable. If Merkel is willing to let Germany's pre-eminent position in Europe do the talking for her, Trump wants to push America's No. 1 position on the global stage --and will take care of all the talking too.
Merkel and Trump have had a rocky relationship since the early days of his presidential campaign, when he claimed she was "ruining Germany" after Time Magazine picked her as the 2015 Person of the Year instead of him.
Their first meeting, which took place in Washington in March 2017, included the moment when Trump appeared to ignore Merkel's offer of a handshake, a much-remarked upon incident. During her next visit to Washington this year, Trump came out with two kisses on the cheek. But the positions articulated by the two on issues ranging from trade and military spending to climate change and migration have remained far apart.
Going into Wednesday's summit, Merkel is emerging from a battle within her government over migration. It remains to be seen if scars will be visible on the international stage.
Trump already pounced on Germany in the run-up, sending a tweet that said, "the United States is spending far more on NATO than any other Country." Then he singled out one: "they must do much more. Germany is at 1%, the U.S. is at 4%."
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THERESA MAY
British Prime Minister May might be even more politically weakened when she encounters Trump. Her week started with her chief Brexit negotiator and her foreign secretary both resigning in disagreement with May's plan for Britain's post-Brexit relationship with the European Union.
May's relationship with Trump started well enough. Soon after his inauguration, she was the first foreign leader to pay an official visit. The two were seen holding hands briefly as they walked along the White House colonnade. The so-called "special relationship" between Britain and the United States seemed secure.
Since then, Trump waded feet first into Britain's approach to countering terrorism after a string of attacks, upsetting both May and the mayor or London with tweets that came off as put-downs. May last month called Trump's decision to impose punishing tariffs on steel and aluminum imports from the EU "unjustified and deeply disappointing."
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EMMANUEL MACRON
After that handshake in Brussels, the Trump-Macron relationship really took off. The two stayed in touch by phone and shared embraces and kisses when they met again in April.
Macron -- who at 40 is young enough to be Trump's son -- was the first foreign leader treated to a lavish state visit to Washington. In a gesture that would be scrutinized for its meaning, Trump brushed the shoulders of the French president's pristine suit before one of their photo ops.
"We have a very special relationship; In fact, I'll get that little piece of dandruff off. We have to make him perfect -- He is perfect," Trump chattered as Macron smiled, appearing unruffled by the act of...intimacy? Dominance? Protectiveness? Disrespect?
Policy-wise, the relationship clearly is imperfect.
Trump threw cold water on the love fest by pulling out of the Paris climate agreement and a nuclear accord with Iran, two initiatives the Obama administration singed onto in 2015 and Macron strongly backs.
Trump's decision to impose the steel tariffs created more distance. Macron called it "illegal" and a "mistake." By June, Macron was looking to take the lead of a European brigade against Trump, rejecting American "hegemony."
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JUSTIN TRUDEAU
The wounds will be still raw when the two meet Wednesday, barely a month after Canadian Prime Minister Trudeau organized the Group of Seven summit of major economies that ended with Trump calling his North American neighbour "dishonest" and "weak" by Trump.
Trump roiled the G-7 meeting in Canada by first agreeing to a group statement on trade and then withdrawing from it. The president complained he had been blindsided by Trudeau's criticism of the U.S. tariff threats. As so many others before him, Trudeau learned of Trump's ire straight from the president's fingers on Twitter.
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THE DONALDS
Relations between Washington and the European Union have sunk so low by now that there will not be a full summit between the two this year.
Trump and EU chief Donald Tusk nevertheless will be in the same room often enough during NATO events this week.
Tusk, a former Polish prime minister who is president of the European Council, let it be known before the 2016 U.S. presidential election that he thought "one Donald is more than enough!" He says time has only proven him more right.
Tusk has described Trump's abrasiveness as a challenge for Europe equal to China's expanding economic power or Russia's belligerence.
On Monday, he called on Trump directly, saying "Dear President Trump: America does not have, and will not have a better ally than Europe" before adding "appreciate your allies, after all you don't have that many."
This spring Tusk said that "Someone could even think 'with friends like that, who needs enemies,"' after Trump first threatened to slap tariffs on EU steel and aluminum exports and reneged on an agreement to stop Iran from developing nuclear weapons.
At the NATO summit, where leaders will discuss how to counter Russian aggression, Trump's one-on-one summit with Russian President Vladimir Putin days after will be the big elephant in the room.
"When you meet President Putin in Helsinki," Tusk said, "it is always worth knowing: who is your strategic friend? And who is your strategic problem?"
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DUSKO MARKOVIC
As for Montenegrin Prime Minister Markovic? No more shoving for a place in the front row. This year, Montenegro will be represented at NATO by President Milo Djukanovic. A former basketball player, Djukanovic is definitely taller than Donald Trump.
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Frank Jordans in Berlin, Dusan Stojanovic in Belgrade, Serbia, and Elaine Ganley in Paris, contributed to this report.
