Germany’s next chancellor, Friedrich Merz, set European pigeons flying in circles when he suggested last month that given rising mistrust in President Trump’s commitment to NATO, he wanted to talk to France and Britain about extending nuclear deterrence over Germany.

Warning that a “profound change of American geopolitics” had put Poland, as well as Ukraine, in an “objectively more difficult situation,” Prime Minister Donald Tusk of Poland suggested the same, while hinting that Poland, with its long history of Russian occupation, might eventually develop its own bomb.

Then Poland’s president, Andrzej Duda, said this week that it was time for the United States to consider redeploying some of its nuclear weapons from Western Europe to Poland. “I think it’s not only that the time has come, but that it would be safer if those weapons were already here,” Mr. Duda told the Financial Times.

The uproar was immediate, given the sensitivity and complications of the nuclear issue and the whole concept of extended deterrence — the willingness of a nuclear-armed country to use its nuclear weapons in defense of a nonnuclear ally. That commitment is at the heart of NATO’s Article Five, promising collective defense, and hinges on the massive American nuclear arsenal.

Mr. Trump and his officials say they remain committed to extending the American nuclear umbrella over Europe, the vital deterrent to any serious Russian aggression, and to the alliance itself. But his evident hostility toward Europe has so unnerved America’s traditional European allies that it has provoked strong doubts that they can depend on the United States.

There are fears that talking too much about a European replacement, let alone trying to construct one, would only encourage Mr. Trump to withdraw his pledge. Even so, European allies are now engaged in the most serious debate in generations about what Europe’s nuclear defense should be.