In an interview with The Telegraph earlier this month, Donald Trump once again threatened to withdraw the United States from the North Atlantic Treaty Organisation (NATO), the collective defence alliance it has been the backbone of for over seventy-seven years. Just a few days earlier, Trump had described the Alliance as “a paper tiger” and slammed Allies as “cowards” in one of his many posts on Truth Social, in response to their initial refusal to assist in securing the Strait of Hormuz as part of the ongoing US-Israeli war of choice against Iran. He is now reportedly even considering punishing measures, such as withdrawing American troops from Germany, suspending Spanish membership and revisiting Britain’s Falklands claim. 

The US President has taken a markedly transactional view of the Alliance, which he had threatened to abandon already during his first term. In 2016, while still a presidential candidate, Trump stated that the US would only intervene to defend those Allies who had met the 2% defence investment pledge agreed at the 2014 Wales Summit. Then, at his first NATO Summit as President in 2018, he threatened to abandon NATO unless Allies met the abovementioned defence spending target by 2024. Trump has also repeatedly questioned the benefits of NATO membership for the US. 

In 2024, again as a presidential candidate, Trump even publicly claimed during a rally in South Carolina to have told Allies he would encourage Russia “to do whatever the hell they want” to those who didn’t “pay their bills”, as he put it. Since moving back into the Oval office, he has pushed Allies to agree at the 2025 Hague Summit to raise defence spending to 5% by 2035. At the same time, he also considered relinquishing to Europeans the position of Supreme Allied Commander Europe (SACEUR), which has been held by an American senior military officer ever since its establishment in 1950. Eventually, however, he settled for handing over leadership of Joint Force Commands (JFCs) Naples and Norfolk, while taking over the Maritime Command (MARCOM). Under this ongoing reform, all three NATO’s operational-level headquarters – JFCs Naples, Brunssum and Norfolk – will be led by Europeans – with all three tactical-level commands – LANDCOM, AIRCOM and MARCOM – led by Americans.

Trump has also considerably shaken trust in the Alliance by insisting on the need for the US to annex Greenland, citing national security concerns and Denmark’s alleged inability to stop Russia and China in the region, and threatening to impose further 25% tariffs on any European Allies that opposed him. 

President Trump’s actions and rhetoric are already significantly damaging the credibility of Article 5, on which the Alliance’s deterrence posture is largely based. However, a sudden and swift US withdrawal from NATO could have disastrous consequences.

Hence the question: can Trump really pull the US out of NATO? 

While the North Atlantic Treaty (NAT) provides for neither a suspension clause nor infringement procedures that the US could activate against other members, it does allow for withdrawal under Article 13 NAT, which, though never triggered, permits any party to leave one year after giving notice of denunciation to the US government as depositary. Unlike Article 11 NAT, however, which explicitly requires that ratification be carried out in accordance with each member’s constitutional processes, Article 13 makes no reference to internal constitutional procedures. Yet it should be noted that Article 11 refers not just to ratification, stating that “This Treaty shall be ratified and its provisions carried out by the Parties in accordance with their respective constitutional processes.” This would seem to include withdrawal as well, although not expressly stated in Article 13. 

The problem is that the constitutional process for treaty withdrawal in the US is not as straightforward as that for ratification. Article 2 § 2, cl. 2. of the US Constitution expressly requires a two-thirds majority in the US Senate for the President “to make Treaties”. However, it is silent on whether the Senate has any formal role when it comes to unmaking treaties. Whether a US President could unilaterally terminate or withdraw the US from a treaty remains a matter of constitutional debate. To complicate matters further in the case of NATO, the US President is also the Commander-in-Chief of the armed forces, which gives the executive branch significant control over military commitments in practice.

Congress has adopted important legislative steps since Trump’s first presidential election victory to restrain the President from abandoning the Alliance. 

Following Trump’s threat to leave the Alliance at the 2018 Brussels Summit, the 2019 NATO Support Act was introduced and passed in the US House of Representatives in January 2019 by a vote of 357 to 22. The bill sought to prohibit the use or appropriation of funds for the purpose of withdrawing from NATO. The bill also stated, inter alia, that Congress did not consider Goldwater v. Carter to be controlling legal precedent with regards to US withdrawal from treaties. In 1979, then-President Jimmy Carter had moved to terminate the 1954 mutual defence treaty with Taiwan without seeking congressional approval, following the shift to the “one China” policy and efforts to normalise relations with Beijing. Senator Barry Goldwater and others challenged the decision, but the US Supreme Court eventually dismissed the case, with a plurality of justices expressing the opinion that it raised a political question or was not ripe for judicial review. The ruling left unresolved  the question of whether a US President may unilaterally terminate treaties, effectively leaving the issue to the political branches to sort out.

The 2019 bill also mentioned that it was the policy of the United States to oppose any efforts to withdraw from NATO, whether directly or indirectly, including by reducing contributions to NATO structures, activities, or operations in a manner amounting to a de facto withdrawal. Although the bill did not advance within the Senate, it was reintroduced in 2023 and later incorporated as an amendment in the National Defence Authorisation Act (NDAA) for the Fiscal Year 2024.

The 2024 NDAA, enacted in December 2023 following signature by then-President Joe Biden, provides in Section 1250A that “the President shall not suspend, terminate, denounce, or withdraw the United States from the North Atlantic Treaty”, nor use funds for that purpose without the advice and consent of two-thirds of the Senate or an act of Congress. 

On this basis, if Trump decided to withdraw the US from NATO, Congress could challenge the decision claiming a violation of Section 1250A of the NDAA. However, in 2020, when Trump withdrew from the Open Skies Treaty, the US Department of Justice’s Office of Legal Counsel published an opinion concluding that by requiring notification of the House and Senate Armed Services Committees, the House Committee on Foreign Affairs, and the Senate Committee on Foreign Relations, 120 days prior to the notice of intent to withdraw from the treaty, Section 1234(a) of the 2020 NDAA unconstitutionally restricted the President’s authority to execute the rights of the United States under treaties and to conduct diplomacy.

It is unclear at this moment in time how the US Supreme Court would rule – would it side with the executive or with the legislative branch? – if it were asked to decide on the merits of a case concerning a President’s decision to unilaterally withdraw from NATO. In reality, the Court is far more likely to avoid the issue altogether, as it has done before in cases like Goldwater v. Carter. This reluctance may be reinforced by the increasingly politicised nature of judicial appointments, which risks undermining the Court’s perceived independence and its willingness to act as a robust check on the political branches.

It is not even clear whether Congress would bring a lawsuit over such a decision, reflecting broader uncertainty about how far the legislative branch is willing or able to escalate inter-institutional conflicts in a highly polarised environment. What is certain, however, is that a decision to leave NATO, whether later challenged or not, would immediately be read in Moscow and European capitals as a decisive break with long-standing US commitments to European security. The strategic signal would land first, with any court rulings coming later, if at all.

But Congressional oversight can play a role in constraining presidential action. Testimony like that of former SACEUR General Christopher Cavoli to the Senate Armed Services Committee last year, warning of the consequences of relinquishing the position of SACEUR, shows that senior military officers can still shape the debate in Washington. Bipartisan Congressional support for NATO, coupled with strong pushback from military and intelligence circles, can materially raise the political cost of withdrawing from the world’s most powerful Alliance, even if it cannot eliminate the existing constitutional uncertainty. The real question is not whether Trump can leave NATO, but whether we are ready to find out.