This deterrent effect doesn’t come just from the NATO treaty, a bare-bones document whose signatories simply agree in Article 5 that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all.” Deterrence comes from the Kremlin’s conviction that Americans really believe in collective defense, that the U.S. military really is prepared for collective defense, and that the U.S. president really is committed to act if collective security is challenged. Trump could end that conviction with a single speech, a single comment, even a single Truth Social post, and it won’t matter if Congress, the media, and the Republican Party are still arguing about the legality of withdrawing from NATO. Once the commander in chief says “I will not come to an ally’s aid if attacked,” why would anyone fear NATO, regardless of what obligations still exist on paper? And once the Russians, or anyone else, no longer fear a U.S. response to an attack, then the chances that they will carry one out grow higher. If such a scenario seems unlikely, it shouldn’t. Before February 2022, many refused to believe there could ever be a full-scale Russian invasion of Ukraine. ... When I asked several people with deep links to NATO to imagine what would happen to Europe, to Ukraine, and even to Taiwan and South Korea if Trump declared his refusal to observe Article 5, all of them agreed that faith in collective defense could evaporate quickly. Alexander Vershbow, a former U.S. ambassador to NATO and a former deputy secretary-general of NATO, pointed out that Trump could pull the American ambassador from his post, prevent diplomats from attending meetings, or stop contributing to the cost of the Brussels headquarters, all before Congress was able to block him: “He wouldn’t be in any way legally constrained from doing that.” Closing American bases in Europe and transferring thousands of soldiers would take longer, of course, but all of the political bodies in the alliance would nevertheless have to change the way they operate overnight. James Goldgeier, an international-relations professor at American University and the author of several books on NATO, thinks the result would be chaotic. “It’s not like you can say, ‘Okay, now we have another plan for how to deal with this,’ ” he told me. There is no alternative leadership available, no alternative source of command-and-control systems, no alternative space weapons, not even an alternative supply of ammunition. Europe would immediately be exposed to a possible Russian attack for which it is not prepared, and for which it would not be prepared for many years.

https://www.theatlantic.com/magazine/archive/2024/01/trump-2024-reelection-pull-out-of-nato-membership/676120/