The visit to Nato headquarters in Brussels – part of the President's first foreign trip – was expected to be thorny given that Mr Trump has praised the Brexit vote for the UK to leave the EU and called the 28-member alliance “obsolete” several times in the past.
There had been conciliatory remarks in recent days about the need for Mr Trump and his Nato allies to work together, particularly in light of the terror attack in Manchester, but Mr Trump pulled no punches in remarks in front of the other leaders and set the stage for a frosty meeting.
As part of the meeting, the 28 member nations, plus soon-to-join Montenegro, will renew an old vow to move toward spending 2 per cent of their GDP on defence by 2024. Only five members currently meet the target: Britain, Estonia, debt-laden Greece, Poland and the United States, which spends more on defence than all the other allies combined – a bugbear of Mr Trump’s both on the campaign and once he was in the Oval Office.
His stance on Nato seemed to soften after personal meetings with German Chancellor Angela Merkel and Nato Secretary General Jens Stoltenberg in Washington.
However, Mr Trump went on the offensive again on Thursday. He said that the payments should be increased by other members in order to “make up for the years lost”.
Jorge Benitez a Senior Fellow at the Brent Scowcroft Center on International Security and Director of NATOSource told The Independent he thinks the defence spending issue may just be a "convenient excuse" Mr Trump is using to rile up his base at home.
He said Mr Trump does not believe in US leadership in an alliance like Nato while governing through his "America First" doctrine.
“I have been very, very direct ... on what 23 of the 28 members ... should be paying for their defence," Mr Trump said.
“This is not fair to the people and taxpayers of the United States,” Mr Trump said, adding that many member nations owe “massive amounts of money from past years and not paying in those past years”.
“If Nato countries made their full and complete contributions, then Nato would be even stronger than it is today, especially from the threat of terrorism,” Mr Trump added.
The spending issue is just one of the issues surrounding the meeting, with many Nato nations concerned by Mr Trump's dallying over his commitment to the alliance's mutual defence pact, known as Article 5.
At the opening of the new Nato headquarters – which included the installation of a piece of the Berlin Wall, and a piece from the World Trade Centre targeted in the 9/11 attacks – Mr Trump offered a vague promise to “never forsake the friends that stood by our side” but did not actively mention Article 5. The 9/11 attacks was the last, and only, time that Article 5 was invoked – with White House officials scrambling later to say that the vague wording from Mr Trump amounted to an affirmation of the defence clause.
Asked about Mr Trump not explicitly affirming US support for Article 5, White House press secretary Sean Spicer said: “It goes without saying. His presence at this event underscores our commitments and treaty obligations.”
Mr Benitez, who is in Brussels attending the summit, said the mood in the halls of headquarters did not reflect Mr Spicer's statement.
"If you are a strong supporter of Nato it is a no-brainer" to vocally support Article 5, said Mr Benitez. In that respect he feels Mr Trump has at least been consistent.
Mr Benitez said the president should have been "clear, transparent, and direct...to reassure our allies" about Article 5 since he was speaking in front of a memorial to the very tragedy that prompted it.
It is a symbol that an attack on ally is an attack on all Nato allies. "The US is now not a reliable ally" he said.
European and Nato allies of the US have also sought clarification over other issues – including the Paris global climate change agreement which Mr Trump has not ruled out withdrawing from – and the actions of Russia and the potential threat to nations near the country's borders.
On Thursday morning, Mr Trump met with European Council President Donald Tusk, who said although the two agree that efforts to fight terror groups like Isis should be increased – with Nato allies set to agree to join the US-led coalition against the terror group – there were still major differences on important topics.
“Some issues remain open, like climate and trade. And I am not 100 per cent sure that we can say today – we means Mr. President and myself – that we have a common position, common opinions about Russia,” said Mr Tusk.
Mr Trump then had lunch with newly elected French President Emmanuel Macron, who has been a critic of Mr Trump.
The pair's handshake showed the already strained relationship, with their knuckles turning white, their jaws clenching, and faces tightening. Mr Macron has made clear he will talk to Mr Trump about the climate change issue.
The meeting with EU leaders, and the lunch with Mr Macron gave Mr Trump a taste of what to expect from the working dinner late on Thursday – with Mr Trump's initial remarks perhaps showing Nato leaders the same back.
That continued at the “family photograph” the leader's gathered for after Mr Trump's speech – with Mr Trump standing alone as other leaders chatted among themselves.
Even without the various issues at stake, Mr Trump's welcome in Belgium had been frosty, with thousands of protesters turning up when he arrived on Wednesday, and a number also turning out ahead of the meeting on Thursday. Mr Trump has previously described Brussels as a “hellhole”.
Part of the problem is that Mr Trump does not fully understand the alliance because of his complete lack of foreign or domestic policy experience, said Mr Benitez.
"I think it's an attitude problem for him," he explained.
The lack of policy experience withstanding, Mr Benitez thinks this summit is "a failure" for the Trump administration because "this was not a policy meeting as much as a meeting about personal relationships."
Mr Trump's tense moment with Mr Macron and how he appeared to shove Montenegro Prime Minister Dusko Markovic out of his way while walking with the other leaders have been noticed by other leaders and their staffs.
There is going to be "great disappointment" for US allies especially those geographically close to Russia, said Mr Benitez.
"This is a Nato victory" for Russian President Vladimir Putin, "doubt and uncertainty...all these things help Russia," he said.
However, it was obvious to all that progress needs to be made – with Nato Secretary General Jens Stoltenberg trying to hammer home the message of unity.
“Nato is more than a club, more than an organisation. Nato embodies the unique bond between Europe and North America,” Mr Stoltenberg said. “As we raise our flags today, our alliance stands strong united and resolute,” he said.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies