Further protests were planned on Thursday, when Trump will first meet with EU President Tusk and Commission chief Juncker at the bloc's headquarters."The NATO of the future must include a great focus on terrorism and immigration, as well as threats from Russian Federation and our nation's eastern and southern borders", Trump said.
The American people and the world are again faced with the variance between what Trump said as a candidate and what he says as president.
Rep. Steny Hoyer of Maryland, the second-ranking Democrat in the House of Representatives, said in a statement that Trump "should not be lecturing our closest and most steadfast allies".
Last year, Trump threatened to abandon US allies in Europe if they did not spend enough on defense, comments that were particularly unnerving for the ex-Soviet Baltic states on Russia's border which fear Moscow might try a repeat of its 2014 annexation of Ukraine's Crimea.
"Twenty-three of the 28 nations are still not paying what they should be paying and what they're supposed to be paying for their defense", Trump said.
Trump refused to say he would adhere to the mutual defense pact, known as Article V, though the White House later claimed that his very presence alongside twisted World Trade Center steel - a memorial outside North Atlantic Treaty Organisation headquarters - was evidence enough of his commitment.
Mr Trump, on Thursday, insisted it was unfair for the United States to shoulder the financial burden as "23 out of the 28" members refuse to spend at least two per cent of their GDP on defence.More news: Ariana Grandes boyfriend cancels gigs after Manchester attack
More news: Sarfraz Ahmed says Pakistan have nothing to lose in Champions Trophy
More news: Fans, mourners gather to honor Chris Cornell
And the world leaders apparently know this and may water down the final communique, which would be significantly different to the one unveiled at its last summit in Japan.
It would have been better if Trump had coupled that message - pay up, folks - with a more explicit expression of the United States' commitment to the alliance.
Does NATO see eye-to-eye on what the threats are and the priorities are today?
On Thursday, Mr Trump brought the "America first" themes of his presidential campaign to the very heart of Europe.
Once more, he will likely be received warily, a president who ran on a campaign of "America First" with suggestions of disentangling the United States from global pacts, now engaged in two days of pomp and policy with the leaders of the United Kingdom, France, Germany, Italy, Japan and Canada. We will never forsake the friends who stood by our side.
His pending review of USA climate policies and decision not to make up his mind before Taormina has braced environmentalists for the possibility of bland language that says little after years of increasingly stronger commitments to reduce reliance on fossil fuels and emissions of in greenhouse gases under the Paris Agreement.