Although administration officials point to the long-standing alliance between the United States and the United Kingdom, Mr. Trump's itinerary will largely keep him out of central London, where significant protests are expected.
"The question [on] all sides is not just a tactical one, that is, how to resolve a disagreement, but a strategic one", says Stephen Sestanovich, a senior fellow for Russian and Eurasian studies at the Council on Foreign Relations. Unlike many other news organizations, we have not put up a paywall.
Trump has described the spending situation as "disproportionate and not fair to the taxpayers of the United States".
This means that Trump is not able to use his executive power to pull America out of the alliance, in contrast to other global agreements such as the Iran nuclear deal or the Paris climate agreement, both of which Trump has withdrawn from.
At the end of the first day, Trump unsettled allies even more by questioning the value of the military alliance, which was formed to counter Soviet aggression during the Cold War. "It has been really awesome to see the level of spirit in that room". They warned nations that invest with Russian Federation could face United States sanctions for doing so.
Trump on Wednesday urged members of the alliance to meet their commitment of at least 2 percent of their gross domestic products by 2024, and eventually 4 percent. The figure is non-binding, and the goal was to reach it by 2024, not immediately.
"Now we have to rebuild that democracy and strengthen it. North Atlantic Treaty Organisation is going to take a significant role in that, and Canada is going to commit 250 troops, a number of helicopters, and we are actually offering to command that mission for the first year", Canadian Prime Minister Justin Trudeau announced at the summit.
He also said meeting the queen would be an experience Trump "will really cherish". "There is a communique that was published yesterday".
"As with all our summits, sometimes the corridors, comments and tweets take on more importance than what is negotiated, said or endorsed by heads of state", Macron said.
"The language was much tougher today", a source told Reuters.
"President Trump wants to see our allies share more of the burden and at a very minimum meet their already stated obligations,"said Sanders. Germany is contributing troops as a lead nation in Lithuania and NATO's forward presence to the east", she said".
"President Trump never at any moment, either in public or in private, threatened to withdraw from NATO", French President Emmanuel Macron said.
Asked if he would change his tune and tweet something else as he headed to England aboard Air Force One, Trump said, "that's other people that do that". "I'm very consistent. I'm a very stable genius".
"I have to tell you that the atmosphere last night at dinner was very open, was very constructive and it was very positive", Kolinda Grabar-Kitarovic, the president of Croatia, told reporters.
Mr Trump also spent time with Turkish president Recep Tayyip Erdogan, who has recently been reelected and gained new powers as his country's executive president.
Montana GOP Sen. Steve Daines suggested that Jon Huntsman, the USA ambassador to Russian Federation, should be highly involved in the discussions.