In some senses, this community has never been stronger. Ever since President Barack Obama went to Tallinn, Estonia, in 2014, in the wake of the Russian invasion of Ukraine, and declared that “the defense of Tallinn and Riga and Vilnius is just as important as the defense of Berlin and Paris and London,” the alliance has been increasing its exercises, broadening its scope. There are British, German and Canadian troops deployed to the Baltic states and U.S. troops in Poland. All of these missions are going well. In public, you will rarely hear a NATO general or government official question the health of the alliance or of the community more broadly.

But under the surface, the world’s largest alliance is extremely uneasy. And it is uneasy, to put it simply, because the U.S. president makes it uneasy. This is not because President Trump behaves inappropriately, inventing lies about Robert S. Mueller III and insulting House Speaker Nancy Pelosi (D-Calif.) while speaking against the backdrop of a Normandy cemetery; it’s not because he wants U.S. allies to spend more money on defense, since most of the people at places like RUSI agree on that. It’s because he uses the same isolationist language as the Americans who opposed entry into World War II — the same Americans who, if they had won the argument in the 1940s, would never have allowed D-Day to happen at all.

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In April 1941, Charles Lindbergh gave a speech at a meeting of the America First Committee, during which he called on Americans to stay out of the war, so that they might “contribute to the progress of mankind in a more constructive and intelligent way than has yet been found by the warring nations of Europe.” Hundreds of thousands of Americans, including future presidents Gerald Ford and John F. Kennedy, initially agreed and joined the America First movement. But that was before Pearl Harbor. Trump, by contrast, agrees with that analysis in retrospect, decades after the war’s end. In a book he published in 2000, Trump — who also likes the expression “America First” — wrote that in Europe, “America has no vital interest in choosing between warring factions whose animosities go back centuries. . . . Their conflicts are not worth American lives.”

If NATO were an old-fashioned sort of alliance, Trump’s language might not matter. But NATO is a defensive alliance: It exists not to attack but to deter, precisely so that we don’t have to fight another battle as bloody as D-Day. And the deterrent effect works because Article 5 of the North Atlantic Treaty implies that the United States, and its nuclear arsenal, would come to the aid of a NATO member under attack. Uniquely, the U.S. president can put that guarantee in doubt — just by talking. And Trump does like to talk, frequently and irresponsibly, about how much he dislikes Europe.

Europeans do not respond much in public. But in private, they talk about how long he will last, how much damage he will do and who will come next. Different European capitals have made different assessments. In France, discussions of a post-NATO future have, tentatively, begun. In London, anyone who suggests there might be an alternative to NATO is dismissed as naive. In Berlin, most people seem to be hoping that things will revert to normal when Trump finally departs.

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There are good reasons to hope for the best: Polls show that Americans not only support remaining in NATO but also support trade agreements and, more generally, the United States’ international commitments. Indeed, they show that this support has risen since Trump became president — perhaps as a reaction to the same language that the United States’ allies find so unnerving.

Still, isolationism was the dominant foreign policy philosophy in the United States between World War I and World War II, and it would be foolish to imagine it can’t ever become so again. The benefits of the United States’ 75-year engagement with Europe are now so much a part of the bedrock that most people take them for granted. The decades of peace; the vast flows of mutually beneficial trade; the ease of travel and exchange; the easy conversations between soldiers of different nations; the fact that all of this was made possible by the D-Day troops, and that all of it remains possible thanks to their modern descendants, is what the president doesn’t seem able to recognize. And if he can’t, then others won’t, either.