April 8 (UPI) — NATO Secretary General Mark Rutte met with U.S. President Donald Trump on Wednesday as he kicked off a visit to Washington amid growing tensions between the American leader and the defensive military alliance.

Trump, a longtime NATO critic, has increasingly expressed frustration with alliance partners, threatening to pull the United States out of the pact over their reluctance to back the U.S. military in the war against Iran.

Following the closed-door meeting with Rutte, Trump lashed out online.

“NATO WASN’T THERE WHEN WE NEEDED THEM, AND THEY WON’T BE THERE IF WE NEED THEM AGAIN,” Trump wrote on his Truth Social media platform.

“REMEMBER GREENLAND, THAT BIG, POORLY RUN, PIECE OF ICE!!”

Two days ago, Trump told reporters at a press conference that his current frustrations with the alliance started with their refusal to hand over Greenland to the United States.

“It all began — if you want to know the truth — Greenland,” he said. “They don’t want to give it to us, and I said, ‘bye-bye.'”

Trump had argued U.S. ownership of Greenland is necessary for national security and has threatened to take it by force if necessary. European leaders contended that Greenland, a territory of alliance partner Denmark, is a part of NATO and home to a U.S. military base.

Rutte had been expected to use the meeting to try to smooth over trans-Atlantic tensions stoked by the refusal of some NATO countries to let U.S. military planes use their airspace or bases.

Allies also declined to take part in military action to reopen the Strait of Hormuz, effectively blockaded by Iran since the start of the war on Feb. 28, and to provide naval vessels to escort oil and gas tankers through the key sea lane.

NATO Spokesperson Allison Hart confirmed that Rutte and Trump had met at the White House, stating they had “a frank discussion on a range of issues related to our shared security, including in the context of Iran.”

“The secretary general underscored the importance of Allies continuing to step up to deliver a stronger, fairer alliance,” she said in a brief social media statement.

Speaking with CNN after the meeting, Rutte said that Trump was “clearly disappointed with many NATO allies,” but he attempted to show the American leader that most were supportive of his aims and actions.

Rutte said he could see Trump’s perspective, but said he told Trump that the “large majority” of European nations assisted with basing, logistics and other support.

Rutte sidestepped answering whether Trump discussed withdrawing the United States from the 32-member defense alliance by crediting Trump’s leadership for the recent commitment from all NATO partners to invest 5% of gross domestic product in their own defense, up from the previous pledge of 2%.

“Which is a transformational change for NATO, a transformational legacy he leaves behind,” he said.

Rutte also acknowledged that the world was safer after the United States attacked Iran’s nuclear facilities and degraded its conventional military capabilities.

“And this is, by many in Europe, acknowledged,” he said.

Trump’s frustration with what he views as a NATO relationship that is unfairly weighted in European allies’ favor boiled over last week after the spat over the strait, with him questioning the point of U.S. membership and saying he would rethink how much the United States contributes to the alliance going forward.

Ahead of the meeting on Wednesday, White House Press Secretary Karoline Leavitt read to reporters a direct message on NATO from Trump: “They were tested and they failed.”

Leavitt described the actions of some member nations as NATO turning their backs on the American people, whom she said were funding their defense.

Secretary of State Marco Rubio has also adopted an increasingly hawkish stance, calling it a “one-way street” where the United States was always there for other NATO members but was told ‘no’ when it needed to use their military bases, begging the question why it was in the alliance.

The position of European NATO allies is that they were not consulted before the United States launched its airborne offensive in Iran — with the majority of states were not even informed beforehand — and that as a purely defensive alliance, the action has no relevance to NATO.

Former U.S. Ambassador to NATO Kurt Volker told Euronews that vocal criticism of military action against Iran expressed by countries including Germany, France, Spain and Finland ran counter to their own interests.

“The messaging from Europe has been terrible,” said Volker, explaining that it created an opening for Trump to deflect blame onto partners who refused assistance, if his Iran gambit backfires.

“The Europeans could have said, ‘we all have a stake in this and let’s see how we can help,'” added Volker who said this could have been achieved without getting pulled into direct military confrontation with Iran.

Patrick Bury, senior associate professor in Warfare at Bath University, said Rutte had a delicate balancing act to perform of persuading Trump of the alliance’s value while as diplomatically as possible defending members’ right to stay out of the war

“His job is to keep the U.S. in NATO. He represents the alliance as a whole, rather than individual member states,” said Bury.

Matthew Kroenig, vice president and senior director of the Atlantic Council’s Scowcroft Center for Strategy and Security, said if any one could talk Trump down it would be Rutte, calling him a “Trump whisperer.”