Since moving into the White House almost 18 months ago, the US 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 year's 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.
The dynamics between Trump and heads of government with their own predilections could create some volatile chemistry at the two-day summit.
Merkel and Trump are a study in contrast. The German chancellor is as careful with her policies and public statements as the US leader is brash and unpredictable. If Merkel is willing to let Germany’s pre-eminent position in Europe do the talking, Trump wants to push America’s No 1 position on the global stage - and will take care of all the talking too.
Their first meeting, in Washington in March 2017, included the moment when Trump appeared to ignore Merkel’s offer of a handshake, a much-remarked on 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.
Trump already pounced on Germany in the run-up, sending a tweet that said, “the US 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 US is at 4%.”
British Prime Minister Theresa May might be even more politically weakened when she encounters Trump. Her week started with her chief Brexit negotiator and her foreign secretary resigning in disagreement with her plan for Britain’s post-Brexit relationship with the EU.
May’s relationship with Trump started well. 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 US seemed secure.
Since then, Trump waded feet first into Britain’s approach to countering terrorism after a string of attacks, upsetting May and the mayor of 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”.
After that handshake in Brussels, the Trump-Macron relationship took off. The two stayed in touch by phone and shared embraces and kisses when they met again in April.
Macron was the first foreign leader treated to a lavish state visit to Washington. In a gesture that would be scrutinised 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?
The wounds will be still raw when he meets Justin Trudeau barely a month after the Canadian Prime Minister organised the Group of Seven summit of major economies that ended with Trump calling his North American neighbour “dishonest” and “weak”.
Trump roiled the G-7 meeting 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 US tariff threats.
As so many others before him, Trudeau learned of Trump’s ire from Twitter.
Relations between Washington and the EU have sunk so low by now that there will not be a full summit between the two this year. EU chief Donald Tusk, a former Polish prime minister who is president of the European Council, let it be known before the 2016 US presidential election that he thought “one Donald is more than enough”. He says time has only proven him more right.