But French president Emmanuel Macron denied any change in policy as a result of Mr Trump's involvement, saying the allies had simply reaffirmed their earlier commitment to increase defence spending to two per cent of gross domestic product by 2024.
Brooke Singman is a Politics Reporter for Fox News.
The unpredictable US president - and his angry demand that allies such as Canada increase their defence spending - was very much the focal point during this week's gathering in Brussels, with memories of last month's disastrous G7 meeting in Quebec still fresh in the minds of many.
Mr Trump pumps his fist during his arrival in Regent's Park, London, to spend the night at the residence of the US Ambassador. May's leading rival, referring ominously to the "turmoil" that the prime minister faces in her party.
'I was very saddened to see he was leaving government and I hope he goes back in at some point.
"I have a lot of respect for Boris".
"The Prime Minister has a good relationship with the President".
In the same breath, he sounded a strongly conciliatory tone with the Russian leader despite the U.S. intelligence community's assessment that Putin not only meddled in the 2016 elections to help Trump but also continues his cyberwar efforts going into the 2018 mid-terms.
China vows retaliation for US$200B US tariff threat
While China has long sought foreign energy providers to fuel its massive growth, it's becoming more self-sufficient. Benjamin Tal, Deputy Chief Economist for CIBC, said he can't see any scenario where Canada is winning.
"And no, a joint statement is not an obligatory attribute of such meetings". In June, after Trump called for Russian Federation to be readmitted to the G7 (after being booted over Crimea), Obama national security adviser Susan Rice called Trump's comments "very destructive".
Although Trump is staying at the US ambassador's London mansion during his two-day trip to London, all of his scheduled public events are outside the city.
"Dear @realDonaldTrump. US doesn't have and won't have a better ally than EU".
"It's a horrific thing being done where you're feeding billions and billions of dollars from Germany. into the coffers of Russian Federation when we're trying to do something so that we have peace in the world".
He also said he thought spending of 4 percent on defence - similar to the U.S. level - would be the right level. I don't think he'll have any 'Gee, I did it, I did it, you got me, ' Trump predicted of his meeting with the former KGB colonel.
This, from someone who has never criticized Russian Federation for its seizure and annexation of the Crimean peninsula in the Ukraine, and denying that their troops were in Eastern part of that country.
NATO Secretary General Jens Stoltenberg was forced to call an emergency budget meeting of all the allies to deal with the staggering demand.
It's a typically Donald Trump approach to diplomacy and getting things done - he likes to sign pieces of paper that are largely symbolic, he attacks allies on Twitter but keeps sweet in person - and he sabre-rattles using the gravitas of the United States to get his way.
Still, members criticized Mr. Trump for his divisive remarks, saying that they weakened the alliance.