"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 defence", the president said as fellow leaders looked on grim faced.
In a speech at the opening of NATO's new headquarters, Trump slammed USA allies for not spending enough on defense before an uncomfortable-looking audience of fellow heads of state, and in between sparked awkward run-ins with other leaders that underscored strained U.S.
Trump's pending review of US climate policies and decision not to make up his mind before the Taormina summit 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 under the Paris Agreement. "As for those allies he spent the campaign offending ..."
"We felt that there is too much divergence in policy and practice between these two large economies", he said. "Terrorism must be stopped in its tracks or the horror you saw in Manchester, and so many other places, will continue forever".
"The NATO of the future must include a great focus on terrorism and immigration as well as threats from Russian Federation and NATO's eastern and southern borders", he said at the unveiling.
Praise was always going to be in short supply after Mr Trump's sharp election campaign criticism of the alliance, which he blamed for not doing more to combat terrorism.
German Chancellor Angela Merkel struck an entirely different note as she unveiled a memorial made up of a section of the Berlin Wall to mark the end of the Cold War. He also said his allies, who were stood in front of him, should spend more on collective defence.
After receiving warm receptions in Saudia Arabia and Israel, Trump's confrontational stance with long-standing partners in Europe cast a cloud over the meeting in Taormina, where leaders are due to discuss terrorism, Syria, North Korea and the global economy. Article 5 has been activated only once - after the 9/11 attacks.
Analyst Thomas Wright of the Washington-based Brookings Institution said Trump's failure to publicly declare this was "shocking and damaging". When Trump greeted Emmanuel Macron, France's new president, they grabbed each other's hands, jaws clenched, in an extended grip that turned Trump's knuckles white.More news: Dead, 1 Injured, After Stabbing In Portland, Ore
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May had previously said she would question the president over leaked security information, which United Kingdom police had shared with U.S. authorities after the Manchester attack.
US President Donald Trump makes his move and overtakes the prime minister of Montenegro.
After meeting with Stoltenberg, Trump met with European Commission leader Jean Claude Juncker, and European Council President Donald Tusk.
As is usually the case, top officials were left to clean up the mess.
'The Germans are bad, very bad, ' Trump told the European Union leaders, according to a German magazine which used the word böse - which can also translate as nasty or evil. See the millions of cars they sell in the US, terrible. "Terrible. We will stop this", he reportedly said.
But Trump pulled no punches when he went on the offensive about the USA's $65 billion goods trade deficit with Germany.
"Of course that partnership is built on trust and part of that trust is knowing that intelligence can be shared confidently, and I will be making clear to President Trump today that intelligence that is shared between law enforcement agencies must remain secure".
Trump seems to be under the impression that his allies made a promise and broke it.