TRUMP: Twenty-three of the 28 member nations are still not paying what they should be paying and what they're supposed to be paying for their defense.
"This will send a strong political message of Nato's commitment to the fight against terrorism", Nato Secretary General Jens Stoltenberg told reporters ahead of Trump's meeting with the alliance's other 27 leaders.
Trump's visit to summit was marked by an embarrassing incident.
Trump had previously backed Britain's vote to leave and said it would be good for both Britain and the EU.
He added that Trump and his top officials had previously voiced Article 5 support, including "personally" with Stoltenberg at their meeting in the U.S. in April.
Other NATO members have a target of 2% of GDP on defense spending, but only a minority have met this target.
The U.S., which has by far the largest military budget in the world, spends well more than that - 3.6 percent of GDP previous year, for instance, according to The Economist. I prefer to have 5,000 troops from the United States in Poland. "Many of these nations owe massive amounts of money from past years", Trump added.
Just as notable as what Trump said, however, is what he didn't say.
Trump is expected to decide later if the United States will stay in the Paris Agreement on climate change and has been soft on Russia's aggression in Ukraine in past remarks.
And he didn't really clarify the situation on Thursday.
In almost 70 years of existence, Article 5 has only been invoked once - September 11, 2001 - and was the catalyst for NATO's involvement in the war in Afghanistan that followed.
He declined to say whether he and Tusk had brought up the Transatlantic Trade and Investment Partnership (TTIP), a proposed EU-US free trade agreement that Brussels and Washington had spent more than three years negotiating.
But that was a historical observation, not a pledge for the future. Now, as president, his remarks didn't explicitly reaffirm America's commitment to it.
Trump also took a dig at the cost of the brand-new building where he was speaking.
"I refuse to do that - but it is attractive", he said. "But it is handsome".
United Kingdom police stopped sharing information about the Manchester suicide bombing with the USA on Thursday after leaks to American media that police said had risked compromising their investigations.
Instead of being insulted, he took the opportunity to thank Trump for supporting Montenegro's membership in North Atlantic Treaty Organisation.
"We will never waiver in our determination to defeat terrorism and achieve lasting security, prosperity and peace", Trump said in the speech before a dinner with leaders.