Ehhhh…I don’t know if I’d call that authoritative.

So, first of all — quite surprisingly to me, when I first learned about it, the question of whether a President may withdraw from a treaty on his own, without going to Congress, is an open question in American constitutional law. It seems like something so important that we’d have ironed it out, but the Constitution never explicitly laid out the terms, and it’s never been specifically answered by the Supreme Court.

The only time SCOTUS addressed this was in Goldwater v. Carter, and there they ruled on a technicality rather than addressing the core question of the President’s powers relative to those of Congress.

Goldwater v. Carter, 444 U.S. 996 (1979), was a United States Supreme Court case in which the Court dismissed a lawsuit filed by Senator Barry Goldwater and other members of the United States Congress challenging the right of President Jimmy Carter to unilaterally nullify the Sino-American Mutual Defense Treaty, which the United States had signed with the Republic of China, so that relations could instead be established with the People’s Republic of China.

Goldwater and his co-filers claimed that the President required Senate approval to take such an action, under Article II, Section II of the U.S. Constitution, and that, by not doing so, President Carter had acted beyond the powers of his office. While dismissing the case the Court left open the question of the constitutionality of President Carter’s actions.

As an interesting note, the UK also had this — as this was also an open question in the British political system — come up quite recently surrounding Brexit, in R(Miller) v Secretary of State for Exiting the European Union. There, courts decided that the Prime Minister had to go to Parliament, couldn’t unilaterally withdraw from the European Union.

But, getting back to the US, a federal law doesn’t address the issue of whether it might be the case that the President can do this regardless of the wishes of Congress. SCOTUS has also found that there are some hard walls separating powers of different branches of the US government, and especially when it comes to foreign affairs, been deferential to the Executive Branch relative to the Legislative Branch. The presidential authority to act in foreign affairs was found to be not dependent upon Congress’s granting him the ability to make use of it. It might be that the President could successfully challenge a law preventing him from withdrawing from NATO without additional Congressional approval as being unconstitutional.

https://en.wikipedia.org/wiki/United_States_v._Curtiss-Wright_Export_Corp.

United States v. Curtiss-Wright Export Corp., 299 U.S. 304 (1936), was a decision of the United States Supreme Court concerning the foreign affairs powers of the president of the United States. It held that the President, as the nation’s “sole organ” in international relations, was therefore innately vested with significant powers over foreign affairs, far exceeding those permitted in domestic matters or accorded to the U.S. Congress.[1] The Court’s majority reasoned that although the U.S. Constitution does not explicitly provide for such authority, the powers are implicit in the President’s constitutional role as commander-in-chief and head of the executive branch.

Curtiss-Wright was the first decision to establish that the President’s plenary power is independent of congressional permission. Sharply criticized by foreign affairs scholars and legal historians, Curtiss-Wright remains relevant as the legal precedent for further expansions of executive power in the foreign sphere.[2]