If Trump left NATO, the alliance would be ‘fundamentally transformed,’ experts say
Having created the treaty alongside Canada, the United States is deeply embedded in NATO’s structure in ways that would be difficult to unwind without fundamentally reshaping how the alliance operates, experts say.
President Donald Trump’s public frustration with the North Atlantic Treaty Organization, or NATO, over the refusal of fellow members to support the U.S. war effort has reached boiling point, with the president issuing threats to leave the transatlantic military alliance.
The alliance of 32 countries, which Trump has described as “severely weakened and extremely unreliable” in a Truth Social post, was established in 1949 at the end of World War II to provide for the collective defense of North America and Europe.
The president has referred to the security coalition as a “paper tiger,” prompting speculation over whether the U.S. will abandon its long-held leadership role in the organization.
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What would happen if the U.S. left NATO? Having created the treaty alongside Canada, the United States is deeply embedded in NATO’s structure in ways that would be difficult to unwind without fundamentally reshaping how the alliance operates, experts say.
Without the world’s superpower providing the bulk of its military firepower and command structure, its combined military strength would be significantly weakened, its political cohesion strained and its deterrent power called into question, they say.
Plus, the U.S. acts not only as a member of the alliance but also as its “depositary,” meaning it is responsible for maintaining the treaty documents and processing formal notices of accession and withdrawal.
“Practically speaking, it would drastically change the alliance, and it’s very difficult to predict what that would look like,” Julie Garey, a Northeastern University associate teaching professor of political science, said of any potential U.S. withdrawal from NATO.
The alliance would not collapse overnight. Under the terms of the eponymous treaty that established NATO, member states must adhere to a formal mechanism to leave, as outlined in Article 13, though it has rarely been tested in full, Garey said. France is the closest precedent, she said, having exited the alliance’s military command structure in 1966. It fully reintegrated in 2009 under then-President Nicolas Sarkozy.
A U.S. withdrawal could follow a similar, if complicated, procedural path. But the U.S. would have to seek internal approval for the move, according to Fiona Creed, associate professor at Northeastern who formerly served as executive director of the United Nations Association of Greater Boston.
The National Defense Reauthorization Act, signed into law in 2023 by former President Joe Biden, forbids any president from pulling out of the alliance without a two-thirds vote of the Senate or approval from Congress. The U.S. Constitution is clear that treaties require Senate approval — “advice and consent” — to join, but it is silent on how to break them.
If Congress approved the NATO exit, the U.S. would have to give one year’s notice, said Creed. But it would still be bound by Article 5, the provision requiring all members to come to the defense of one another in the event an ally was attacked or otherwise needed reinforcements during that period.
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Europe has already stepped up its defense spending in response to Russia’s war with Ukraine and growing uncertainty about long-term U.S. security commitments. European NATO members are now spending over $450 billion a year on defense, nearly double 2022 levels.
And the political will to invest more in defense has meaningfully shifted, Creed said. Germany, for example, has loosened its constitutional debt rules to allow significant investments in its national defense, and Poland’s defense spending is approaching 5% of gross domestic product, or GDP.
Additionally, the EU’s ReArm Europe initiative has EU members looking to mobilize up to 800 billion Euros for the continent’s defense. Separately, in 2025 NATO leaders agreed to increase defense spending up to 5% of each country’s gross domestic product by 2035, an effort to prepare for a potential future without U.S. military support in the face of Russian aggression.
Still, “What Europe cannot quickly replace is the invisible architecture that the US provides, things like satellite surveillance, missile defense, strategic airlift and integrated command,” Creed said.
The current threat of a U.S. departure, and more broadly, a shake-up in the alliance has long loomed, experts say. The treaty itself is short, just some 2,000 words, and somewhat vague and open to interpretation. It lays out broad principles of collective defense, but not very detailed contractual obligations, Garey said.
Symbolically, a U.S. departure would be a major blow to its global credibility and leadership, experts say.
“It would be the most significant voluntary reduction of American influence since the [U.S.] Senate rejected the League of Nations in 1919,” Creed said, referring to the post-World War I international body that preceded the United Nations.
He added that NATO is more than just a defensive alliance. “It represents a commitment the United States made after World War II that its security depends on the security of democratic Europe,” he said.
If the U.S. were to leave, could it rejoin under a different administration?
It’s certainly possible, experts say. But given the stakes, Europe might be reluctant to tie its security back to the United States so quickly, Garey said, “especially if they see the risk of another reversal in a few years.”
