WASHINGTON — President Donald Trump on Tuesday injected some uncertainty over whether the U.S. would abide by the mutual defense guarantees outlined in the NATO treaty as he headed to its summit — comments that could revive longstanding concern from European allies about his commitment to the military alliance.

“Depends on your definition,” Trump told reporters on Tuesday as he was headed to The Hague, where this year’s summit is being held.

“There’s numerous definitions of Article Five, you know that, right? But I’m committed to being their friends.”

Asked later to clarify, Trump said he is “committed to saving lives” and “committed to life and safety” but did not expand further, saying he didn’t want to elaborate while flying on an airplane.

The remarks, which came to reporters on Air Force One as Trump traveled to the Netherlands, previewed what could be another volatile appearance by Trump at a summit celebrating an alliance that the Republican president has often derided.

And it comes amid a backdrop of tumult in the Middle East, after Trump moved to strike three nuclear enrichment facilities in Iran as well as the president’s sudden announcement that Israel and Iran had reached a “complete and total ceasefire.”

The sharp U-turn in hostilities — followed hours later by Trump’s declaration that both parties violated the agreement — had already started to shape the summit, with NATO Secretary-General Mark Rutte publicly dancing around the issue even as hundreds of people showed up in The Hague on Sunday to denounce the conflict in a protest that was initially focused on defense spending.

Still, other NATO countries have become accustomed to the unpredictable when it comes to Trump, who has made no secret of his disdain for the alliance, which was created as a bulwark against threats from the former Soviet Union.

Trump’s debut on the NATO stage at the 2017 summit was perhaps most remembered by his shove of Dusko Markovic, the prime minister of Montenegro, as the U.S. president jostled toward the front of the pack of world leaders during a NATO headquarters tour.

And he began the 2018 summit by questioning the value of the decades-old military alliance and accusing its members of not contributing enough money for their defense — themes he has echoed since. In Brussels, Trump floated a 4% target of defense spending as a percentage of a country’s gross domestic product.

“NATO was broke, and I said, ‘You’re going to have to pay,’” Trump said Tuesday, recounting his initial encounters with the alliance. “And we did a whole thing, and now they’re paying a lot. Then I said, ‘You’re going to have to lift it to 4% or 5%, and 5% is better.’”

That 5% figure is “good,” Trump said, adding: “It gives them much more power.”

Sen. Jeanne Shaheen of New Hampshire, the top Democrat on the Senate Foreign Relations Committee, will also attend the NATO summit this week.

She said if Trump does anything to sow division within the alliance, it would benefit Xi Jinping of China.

Trump heavily telegraphed his attitude toward global alliances during his presidential campaigns.

As a candidate in 2016, Trump suggested that he as president would not necessarily heed the alliance’s mutual defense guarantees outlined in Article 5 of the NATO treaty.

And during a campaign rally in 2024, Trump recounted a conversation with another NATO leader during which Trump said he would “encourage” Russia “to do whatever the hell they want” to members who weren’t meeting the alliance’s military spending targets.