Long before Donald J. Trump threatened over the weekend that he was willing to let Russia “do whatever the hell they want” against NATO allies that do not contribute sufficiently to collective defense, European leaders were quietly discussing how they might prepare for a world in which America removes itself as the centerpiece of the 75-year-old alliance.

Even allowing for the usual bombast of one of his campaign rallies, where he made his declaration on Saturday, Mr. Trump may now force Europe’s debate into a far more public phase.

So far the discussion in the European media has focused on whether the former president, if returned to office, would pull the United States out of NATO.

But the larger implication of his statement is that he might invite President Vladimir V. Putin of Russia to pick off a NATO nation, as a warning and a lesson to the 30 or so others about heeding Mr. Trump’s demands.

His statement stunned many in Europe, especially after three years in which President Biden, attempting to restore the confidence in the alliance lost during Mr. Trump’s four years in office, has repeatedly said that the United States would “defend every inch of NATO territory.” And while a spokesman for the White House, Andrew Bates, denounced Mr. Trump’s comments as “unhinged,” by Sunday morning they had already resonated with those who have argued that Europe cannot depend on the United States to deter Russia.

Charles Michel, the president of the European Council, which comprises Europe’s heads of government and defines their common policies, wrote that “reckless statements” like Mr. Trump’s “serve only Putin’s interest.” He wrote that they make more urgent Europe’s nascent efforts to “develop its strategic autonomy and invest in its defense.”

And in Berlin, Norbert Röttgen, a member of the German Parliament’s foreign affairs committee, wrote on the social media platform X, “Everyone should watch this video of #Trump to understand that Europe may soon have no choice but to defend itself.” He added, “Anything else would be capitulation and giving up on ourselves.”

All of this doubt is bound to dominate a meeting of NATO defense ministers on Thursday in Brussels and then the Munich Security Conference, an annual gathering of national security leaders, on Friday. And while Vice President Kamala Harris and Secretary of State Antony J. Blinken will doubtless use the moment to celebrate the NATO solidarity that has been critical to keeping Ukraine an independent nation two years after Russia’s invasion, any statements they make will almost certainly be met with doubts about what the alliance will look like in a year’s time.

In fact, that re-evaluation has been underway for months, some European diplomats and defense officials say, though they have alluded to it only obliquely in public, if at all.

Germany’s defense minister, Boris Pistorius, has begun talking about how Germany must prepare for the possibility of decades of confrontation with Russia. The departing secretary general of NATO, Jens Stoltenberg, said last week that the alliance had to prepare for a “decades-long confrontation” with Russia.

In a statement on Sunday, Mr. Stoltenberg said, “Any suggestion that allies will not defend each other undermines all of our security, including that of the U.S., and puts American and European soldiers at increased risk.” He added, echoing statements made by NATO members in 2016, “I expect that regardless of who wins the presidential election the U.S. will remain a strong and committed NATO ally.”

Denmark’s defense minister, Troels Lund Poulsen, has said that within three to five years, Russia may “test” NATO’s solidarity by attacking one of its weaker members, attempting to fracture the alliance by demonstrating that others would not come to its defense. “That was not NATO’s assessment in 2023,” he told Jyllands-Posten, a Danish newspaper, last week, calling it “new information.”

At its core, the argument underway in Europe goes to the question of whether members of the alliance can be assured that the U.S. nuclear umbrella — the ultimate deterrent against Russian invasion — will continue to cover the 31 members of the NATO alliance.

Britain and France have their own small nuclear arsenals. If, over the next year, NATO’s European members came to doubt that the United States would remain committed to Article V of the NATO treaty, which declares that an attack on one constitutes an attack on all, it would almost inevitably revive the debate about who else in Europe needed their own nuclear weapons — starting with Germany.

During the last Cold War, that discussion was quite open, in ways that can seem shocking today. Konrad Adenauer, the first chancellor of the Federal Republic of Germany, declared in 1957 that tactical nuclear weapons — the kind Russia has threatened to use in Ukraine — were “no more than the further development of the artillery.” He added, “We cannot, of course, do without them.” In a 1962 meeting he added that the defense of Berlin “must be fought from the very beginning with nuclear weapons.”

For six decades the United States helped tamp down such sentiments by basing American nuclear weapons across Europe. They remain there to this day. But the value of that deterrent came under question as Mr. Trump — publicly and privately — pressed his aides to withdraw from NATO in 2018.

At the time, Mr. Trump’s national security team, including the defense secretary, Jim Mattis, and two successive national security advisers, H.R. McMaster and John R. Bolton, scrambled to keep Mr. Trump from sabotaging the cornerstone of European defense strategy. Their concern was that American influence in Europe would be undermined, and Russia emboldened.

That was, of course, all prior to the Ukraine war. Now the questions that seemed theoretical to Europeans — starting with whether Mr. Putin was prepared to attempt to retake the lands that he believed were rightly Russia’s, back to Peter the Great — seem vivid, perhaps life-threatening.

When Olaf Scholz, the current German chancellor, prepared last week to meet Mr. Biden in Washington, he wrote in The Wall Street Journal that “Russian victory in Ukraine would not only be the end of Ukraine as a free, democratic and independent state, it would also dramatically change the face of Europe.” It would “serve as a blueprint for other authoritarian leaders around the globe.”

In Washington, Mr. Scholz stressed that Germany had now become the second-largest provider of military aid to Ukraine and was part of the European decision in recent weeks to provide $54 billion over the next four years for the country’s reconstruction.

This year, Germany will finally reach the goal of spending 2 percent of its gross domestic product on defense — the goal set for all NATO nations — years later than first promised. The commitments Europe has now made to Ukraine exceed Washington’s current promises, at a moment when it is unclear whether Republicans in Congress will continue to block additional support.

Mr. Trump mentioned none of this in his threatening remarks on Saturday, of course; Europe’s stepping up to the challenge, if belatedly, does not fit his campaign narrative.

But what will resonate in capitals around Europe will be the wording of what he described as an encounter with an unnamed president “of a big country.”

In Mr. Trump’s telling, the leader asked him, “Well, sir, if we don’t pay and we’re attacked by Russia, will you protect us?” And Mr. Trump recalled saying: “No, I would not protect you. In fact, I would encourage them to do whatever the hell they want. You gotta pay.”

The story, which was seen as implausible in many European capitals, was, 75 years into the alliance, a casting of NATO as more of a protection racket than an alliance.

And whether Mr. Trump wins in November or not, the fact that such a vision of NATO has taken hold with a significant number of Americans represents a shift that is bound to affect the view of the trans-Atlantic alliance in Europe for years to come.

Christopher F. Schuetze and Steven Erlanger contributed reporting from Berlin, and Matina Stevis-Gridneff from Brussels.