President Donald Trump on Wednesday threatened to withdraw the U.S. from NATO after its European members refused to help unblock the Strait of Hormuz, as he upped his criticism of the alliance.
Experts say it is not clear whether Trump could act unilaterally to leave the 77-year-old trans-Atlantic coalition, even though he frequently makes major decisions without congressional approval, some of which are held up by U.S. courts.
Here's a look at the issue:
The Constitution specifies that the president has the power to make treaties with the advice and consent of the Senate, provided two-thirds of the 100-member Senate concur.
However, it is silent on treaty withdrawal.
NATO, which includes European countries, the United States and Canada, was formed in 1949 with the aim of countering the risk of Soviet attack and has been the cornerstone of the West's security ever since.
Article 13 of the 1949 North Atlantic Treaty says that any party may withdraw after giving one year's notice to the government of the United States, which will then inform other governments of the "notice of denunciation."
To date, no NATO member has ever rescinded its membership.
In 2023, Congress passed, and then-President Joe Biden, a Democrat, signed into law, legislation barring any U.S. president from suspending, terminating, denouncing or withdrawing the United States from the treaty that established NATO unless the withdrawal is backed by a two-thirds majority in the 100-member Senate.
The legislation was introduced as an amendment to the 2024 National Defense Authorization Act, a massive annual bill setting policy for the Pentagon. The amendment's lead sponsors were Democratic Senator Tim Kaine of Virginia and then-Republican Senator Marco Rubio of Florida.
Rubio, who is now both Trump's Secretary of State and National Security Adviser, said on Tuesday that Washington would have to reexamine its relations with NATO after the Iran war, which began on February 28 with U.S. and Israeli air strikes.
The NDAA amendment also said that no U.S. funds could be spent on a withdrawal from NATO.
Trump has been harshly critical of NATO for years. In 2020, during his first term, the Department of Justice's legal counsel issued an opinion saying that the president - not Congress - has the exclusive authority to withdraw from treaties.
A February 2026 report by the Congressional Research Service said that, if the issue came up in court, the executive branch could cite that opinion and argue that the NDAA amendment is unconstitutional.
Trump told Reuters on Wednesday that he would state in an address to the nation that he was "absolutely" considering withdrawing from the alliance, citing "disgust with NATO." Trump's remarks came just hours after his Defense Secretary Pete Hegseth declined to reaffirm the U.S. commitment to NATO's collective defense.
Experts said this lack of commitment, rather than any law, was the key point.
"If the president and the military are not committed to NATO and European security, then I don't think there's much Congress can actually do to hold that back," said Max Bergmann, a former State Department official who is now director of the Europe, Russia, and Eurasia Program at the Center for Strategic and International Studies.
Under international law, a nation's head of state generally has the authority to withdraw from a treaty, if the treaty allows withdrawal and the nation adheres to the withdrawal process.
U.S. law has been less clear, although presidents have withdrawn from several treaties without Congress' approval, including Trump's 2020 departure from the 35-nation Open Skies treaty allowing unarmed surveillance flights over member countries.
If the issue ends up in the courts, the challenge to Trump's decision would face steep hurdles. Those include establishing who has standing - a personal stake in the outcome - to challenge the withdrawal.
The U.S. Supreme Court, whose conservative majority often rules in Trump's favor, has never heard a treaty withdrawal case on its merits.
