Diplomats fear a rancorous meeting in Brussels could undermine efforts to show unity in the face of the growing threat on the alliance's eastern flank, particularly with Trump set to meet Putin in Helsinki a few days later.
On the other hand, Trump is right that many countries in the global security alliance come "nowhere close to their 2% commitment".
"Is the President just going to focus on defense spending and basically hijack the entire conversation?" says Heather Conley, director of the Europe Program at the Center for Strategic and International Studies. "As far as I'm concerned, a competitor". "Boris Johnson's a friend of mine". "Frankly, Putin may be the easiest of them all".
"I want to be clear: we're absolutely committed to the protection of our allies and to global peace and security", Trudeau said in Riga, adding that Canadian fighter jets would continue to take part in North Atlantic Treaty Organisation air policing mission in the Baltic countries.
"NATO countries must pay MORE, the United States must pay LESS".
On Nato, President Trump's main objection is that a number of member states have not increased their defence budgets to the target of 2 percent of economic output. Unfortunately these traits will undoubtedly be on display when the USA president arrives at the North Atlantic Treaty Organisation summit in Brussels tomorrow.
"Getting ready to leave for Europe". Not fair to the USA taxpayer. On top of that we lose $151 Billion on Trade with the European Union.
But Mrs May's plans for Brexit would tie Britain to the European Union by having a common rulebook on goods making it nearly impossible to have a free trade deal with the United States or other countries.
Tusk responded in kind on Twitter. But Trump's more aggressive approach on the issue has some anxious that he will focus on defense spending at the expense of other discussions.
Last month, the president even wrote letters to a handful of European countries, urging them to up their defense commitments.
The enduring image of May's visit to the White House in January 2017, when she became the first foreign leader to meet the president after he took office, was Trump taking May's hand to help her down the steps of the White House colonnade.
Tusk also sought to "dispel" the argument that the US alone protects Europe, noting that Europe was the first to respond on a large scale when the USA was attacked on September 11, 2001 and has lost 870 soldiers in the ensuing war in Afghanistan.
As European Council president, Mr Tusk serves as the chairman when EU leaders meet at summits.
"It will become increasingly hard to justify to American citizens why some countries continue to fail to meet our shared collective security commitments", Trump said in a letter addressed to Norwegian Prime Minister Erna Solberg seen by Bloomberg News.