Flying to the NATO meeting in Brussels with Trump, U.S. Secretary of State Rex Tillerson said on Wednesday it would be an important step for the organisation to join the U.S. -led, 68-nation coalition.
The senior official said the message from Tusk and Juncker, who will meet Trump before he joins other national leaders for a summit of the NATO defence alliance in Brussels, would be that the European Union and United States are the "pillars of the free world" and must stand united on worldwide affairs, including conflicts in Syria and Libya, as well as North Korea's nuclear programme.
Trump and Michel said they were on the same page when it came to terrorism, once again an immediate topic in Europe in the wake of the Manchester, England attack this week.
France and Germany have led the resistance to United States demands, fearing that the alliance will be sucked into another grinding campaign in the Middle East and could be stuck with a long-lasting responsibility for securing Iraq.
But it was Trump's undiplomatic rhetoric that got the issue to the top of the group's agenda this week, when NATO's members are expected to accept the idea of public report cards to make sure everyone's meeting the requirements of the alliance.
Trump and his White House have long complained about "leakers" they think are trying to undermine his presidency.
That includes highlighting Canada's contributions to the fight against ISIL, but also intelligence-sharing with allies and combating terrorist financing.
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But in remarks that humiliated other leaders, Trump said that "NATO members must finally contribute their fair share and meet their financial obligations".
Trump sat down Thursday with Donald Tusk, the president of the European Council, Jean-Claude Juncker, the president of the European Commission, and other officials. Leaders will agree to submit annual action plans laying out how they plan to meet NATO's spending goals.
In addition to formally inaugurating NATO's new headquarters, the mini-summit will allow the alliance to assess decisions made at the Warsaw Summit last summer, both in terms of implementation and initial effectiveness.
And while he did say last month that he had changed on his mind on that front after learning more about what North Atlantic Treaty Organisation does, he has never explicitly endorsed Article 5 - the self-defence clause that means an attack on one member generates a response by all.
Trump is in the midst of a nine-day global trip. "The one and only time Article 5 has ever been invoked was on 9/11; that was in the defense of the US", Lewis added.
As President Trump wraps up his first foreign trip as commander-in-chief, one result to watch is whether this nine-day, five-nation swing whets his appetite for future foreign travel.