Here are President Trump's full remarks from May 25 in Brussels. (The Washington Post)

May 25, 2017, will be remembered in Europe as the date on which an American president deliberately went out of his way to avoid an explicit commitment to the principle of mutual defense that is the bedrock of the transatlantic alliance. By all accounts, European leaders had begged the White House to include it. The White House had told reporters it would be.

And then it wasn’t. Instead, President Trump — standing in front of a memorial to 9/11, the only time after which Article 5 was ever invoked (and by the Europeans, on behalf of the United States) — harangued his peers about their defense expenditures. Some — including my friend and Brookings colleague Michael O’Hanlon — have called this a “minor mistake.” I respectfully, but vehemently, disagree. It is one of the most damaging things an American president has ever done to NATO. It amounts to an abandonment of the alliance.

True, the NATO Treaty’s Article 5, which states the commitment to collective defense, is ambiguous, despite its ringing proclamation that an “attack against one … shall be considered an attack against them all.” It gives allies interpretive leeway to decide what they deem to be “necessary, including the use of armed force,” to respond to an armed attack. It kept German chancellors sleepless throughout the Cold War.

The changing nature of today’s threat environment has made the promise even less clear. Instead of enemy tanks at our borders, we are grappling with propaganda, disinformation, corruption, support for political extremists and cyber-hacking: measures that probe our vulnerabilities with a deeply malignant intent yet fall short of an overt attack and are often difficult to attribute with total certainty to a government. Still, European leaders and their intelligence services — and indeed, the U.S. intelligence services — have made it pretty clear that they have good reason to think the Kremlin is behind most of this.

It’s also true that so far none of this is a case for Article 5. Instead, we’ve been reassuring our easternmost allies — the three Baltic republics and Poland, in particular — that we are ready to defend them by deploying thousands of armed, combat-ready NATO troops to their borders and letting NATO planes scramble to the skies when Russian fighters buzz NATO airspace. Those troops, and the planes, are European and American.

We’ve also been trying to address our own vulnerabilities: by improving our cybersecurity, countering propaganda, ferreting out corruption, and, yes, considerably increasing our defense spending. Even Sweden, which works closely with NATO despite its neutrality (but poll support for joining is up), has reintroduced the draft. The sanctions we imposed on Russia remain in place, nearly three years after its annexation of Crimea — a powerful sign of European and transatlantic unity. Our firm support for Ukraine’s right to nationhood and democratic transformation is another measure of our disapproval of Russian aggression.

But none of this means that Article 5 has become irrelevant. Or that the president simply showing up is enough — as White House officials have suggested — to affirm America’s continuing engagement in Europe.

Even more than a willingness to use force in defense of an ally, Article 5 has come to embody the political commitment that undergirds the alliance of the West: an alliance based on shared values and interests and on the protection of a rules-based global order; an alliance based on what we are, and what we stand for, rather than whom we are against. And that is why all presidents since Harry Truman in 1949 have made it a point of honor to reaffirm America’s pledge to NATO. For Trump to refuse to do so is a devastating blow to the alliance’s credibility, at a time when it is surrounded by threats. It increases the risk of a Russian strategic miscalculation, putting American and European soldiers’ lives at risk.

We Europeans do appreciate that some in this administration have been at pains to let us know that on their watch, the United States’ support for the alliance will be unwavering. And we were gratified to hear from the president himself that he no longer considers NATO to be “obsolete.” But it is somewhat less encouraging to know that the United States’ closest allies — the Brits — are incandescent because of White House indiscretions about the identity of the Manchester bomber. It’s duly noted that the president seems far more comfortable with autocrats than with his Western, democratically elected peers. Equivocations about U.S. support for Russia sanctions don’t help.

We know we need to do more for the defense of our alliance. As German Foreign Minister Sigmar Gabriel said to his Russian counterpart, Sergei Lavrov, in Moscow a couple of months ago, we don’t buy the Kremlin’s notion that we live in a post-Western world. We remain committed to the West as a community of values. We will continue to uphold it, with or without our American friends. But we’d prefer the former.