The essence of deterrence is credibility. The best way to stop Russia from attacking even a small NATO country — one of the Baltic states, say — is for Vladimir Putin to be convinced that the risk of major retaliation is too great to take. This is why President Trump’s reaction to the commitment by NATO members at their recent summit to more than double their defense-spending target is welcome, even if a partial opt-out for Spain is not. NATO, said Trump, was not a “rip-off,” high praise from this president. NATO’s European members “really love their countries . . . and we’re here to help them.” Critically, Trump emphasized that the U.S. still regarded itself as bound by Article 5 of the NATO treaty, the mutual-defense provision: “I stand with it, that’s why I’m here. If I didn’t stand with it, I wouldn’t be here.”

Those were welcome words. While no small part of the decision by NATO’s other members to boost their spending can be put down to relentless pressure from Trump, some of the warnings he used to get his way risked creating grave doubts about the extent to which the U.S. stood behind NATO’s mutual-defense guarantees. Such doubts could only embolden Putin and, moreover, encourage NATO members to hedge their bets with Russia, as one or two have already been doing.

Only a few years ago, with many NATO members struggling, or not even bothering to struggle, to reach an earlier target of 2 percent, such a commitment would have been unthinkable. In 2021, just six NATO members out of 32 had hit that target; by 2024, that total had risen to 23. Fierce pressure from Trump 45 had persuaded them that America’s willingness to commit to the defense of an alliance in which most members were not pulling their weight could no longer be taken for granted, whoever was president. And Russia’s war on Ukraine and increasingly menacing behavior toward the broader West had made it obvious this was not the time to chance losing that American support. Last week’s U.S. bombing of three Iranian facilities only underlined that fact.

The new spending target of 5 percent (of which 1.5 percent can comprise defense-related infrastructure spending, a conveniently elastic concept in this case) does not have to be hit until 2035. However, some countries, painfully conscious of geography and history, are racing ahead. Poland is set to spend 4.7 percent of its GDP this year, while Estonia has locked in a program that will mean it should spend 5.4 percent in 2029. No less significant is that Germany — for years one of the worst of the deadbeats, given its economic strength — appears, at last, to give the long-term threat posed by Russia the seriousness it deserves. Berlin will take its spending to 3.5 percent by the end of the decade. Germany’s new chancellor, Friedrich Merz, has declared that the Bundeswehr must “become the strongest conventional army in Europe.”

Merz has also stressed that this spending is not to “do the United States a favor — but because Russia actively threatens the freedom of the entire Euro-Atlantic area,” language designed to rebut any suggestion that the Bundeskanzler is being bullied by Trump. And it makes a more substantive point: A strong NATO is in the interests of the U.S. as well as of Europe, even more so now that the U.S. has to contend with a rising would-be hegemon in the Pacific.

That said, NATO would not have come to this point without U.S. pressure. While, as alluded to above, we have been nervous about some of the tactics Trump has used to this end, there can be no doubt that he (with a major assist from the Kremlin) has done more than any president in decades to persuade the Europeans to accept a much fairer share of responsibility for their own defense. That is a considerable achievement. If the Europeans can deliver on their commitments, the result will be a much safer world. Even if embattled Ukraine saw little direct benefit from the summit, the rebuilding of a stronger NATO will benefit Kyiv as well as the alliance’s members.

Now it is up to Europe to keep those promises. Doubtless, there will be some backsliding — Spain has already been permitted as much, for now. It will be up to this administration and its successors to keep up the pressure on the Europeans to stick to what they have agreed. But they should endeavor to do so in a way that does not undermine the perception — when it comes to deterrence, perception is crucial — of the reliability of the American guarantee, lest they let Putin believe that he has an opportunity to divide and conquer. These are already dangerous times. There is no sense in making them more dangerous still.