He also suggested he could find time to talk to his "friend" Boris Johnson, who has just rocked the Government with his bombshell resignation over Mrs May's Brexit plans, and that he would find it easier dealing with Mr Putin than America's European allies. They got rid of their coal plants, they got rid of their nuclear, they're getting so much of their oil and gas from Russian Federation.
And he vowed not to be "taken advantage" of by the European Union, which he accuses of freeloading by relying on the United States for its defence while restricting USA imports into the bloc, the world's biggest market.
Trump is scheduled to meet Merkel at the summit later Wednesday and with Russian President Vladimir Putin in Helsinki on Monday.
What is the defence spending row about?
While Trump has routinely misrepresented the nature of NATO's funding and how the alliance works to share the burden of mutual defense, he's correct that energy dependence on Russian Federation from Germany, and all of Europe, gives Moscow influence.
In what was supposed to be a brief photo op ahead of a bilateral breakfast meeting, Trump appeared to catch the secretary-general off guard, launching into the broadside as cameras clicked away and Stoltenberg struggled to get a word in.
Before he left Andrews Air Force Base, President Trump reiterated his demand for the U.S. to pay less and other members of the Western military alliance to pay more, and he linked the issue to the EU's trade surplus with the US. However, several, such as Poland and France are close to the mark.
"We all agree that we have to do more", he said, describing previous year as marking the biggest increase in defence spending across Europe and Canada in a generation.
Previous US presidents also urged greater defence spending from European nations.
And she said she expected Trump to recommit to one of the founding articles of NATO - Article 5 - which holds that an attack on one member is an attack on them all. Trump even congratulated Putin for winning re-election earlier this year.
However new figures published Tuesday will do little to satisfy Trump's demands - they show that only seven European NATO countries will reach the two percent target in 2018.
"Dear America, appreciate your allies, after all you don't have that many", Tusk said, before reminding Trump that European troops had fought alongside Americans in Afghanistan following the September 11, 2001 attacks on the United States.
Mr Trump is a hard guest: he's tweeted his meeting with Mr Putin might be easier than those with Europeans.
"The meeting tomorrow will be an opportunity for all of our countries to rededicate ourselves to North Atlantic Treaty Organisation, to demonstrate that it's actually important, that it actually does important things, and that the collection of values and rules that it was brought in to defend 75-plus years ago are just as relevant today as they ever were", Trudeau said. Privately some European diplomats have been shouting - well, raising their voices - at me in back hallways because of their anger over Mr Trump. Others in Europe - like Mr Tusk - are tweeting.
"I would like to thank all our nations for the efforts they are making. I hope that nothing like what happened at G7 occurs", Corker told Reuters. "All countries will be happy", he said, unconvincingly. That is why we have agreed today to strengthen EU-NATO ties in crucially important areas.
European officials say while U.S. defence spending makes up 70 percent of combined allied governments' military budgets, just 15 percent of USA expenditure is spent in Europe on NATO-related defence.
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