President Donald Trump is at it again. Having just arrived in Brussels for the NATO summit, he marked the occasion by launching a full-scale attack on the Germans, whom he promptly accused of being “captive to Russia.” Germany is not just any country, mind you — it’s one of our most important allies, whose troops are still serving by our side in Afghanistan. But given everything that has happened so far, I doubt anyone at the summit was really surprised. On Tuesday, Trump had already sent out the following tweet: “NATO countries must pay MORE, the United States must pay LESS. Very Unfair!”

Foreign policy experts in Washington and other world capitals were outraged. German Chancellor Angela Merkel pushed back against Trump’s comments in unusually forceful terms, noting that “we can make our independent policies and make independent decisions.” But Trump’s fans seem to be eating it up. (That last Twitter comment on NATO got more than 13,000 retweets and almost 64,000 likes.) His attacks on our transatlantic allies certainly don’t seem to have dented Trump’s popularity among his supporters: 87 percent of Republicans think he’s doing a great job.

All of which raises an important question: Does the president’s systematic trolling of our allies reflect the desires of his constituents? Are Americans really sick of NATO and our other international entanglements? Some observers argue that Trump is tapping into long-standing American traditions of isolationism, fueled by a deep suspicion of decadent foreigners and “global elites.” Trump’s fondness for the slogan “America First” traces its roots directly to the isolationist campaigns of the 1930s.

Polls suggest that he’s on to something. In 2013, after years of apparently endless war in Iraq and Afghanistan, Americans’ appetite for overseas intervention plunged to historic lows. Surveys show that large numbers agree with the proposition that U.S. allies aren’t bearing their full share of the costs of their own defense. This sense that our overseas friends are “free-riding” at U.S. expense slots neatly into the populist rage against the status quo that lifted Trump to the White House.

Yet take a closer look at those same opinion surveys and the message becomes more complicated. Solid majorities of Americans routinely register strong support for NATO and our other allies around the world. How does that square with those concerns about burden-sharing?

Security expert Patrick Cronin points out in an interview that transatlantic crises are nothing new. “What’s different about this one is that it’s been generated by deliberate design of the U.S. president,” he says. In the 1950s, President Dwight Eisenhower’s stance on the possible use of nuclear weapons rattled European allies. In the 1970s and 1980s, NATO responded to the stationing of Soviet short-range missiles in Eastern Europe by building up its own missile forces in the United Kingdom and West Germany, to enormous domestic controversy. The Reagan administration even tussled with the Germans over their imports of Soviet natural gas — a fight that presaged Trump’s current nagging about Berlin.

Nor is Trump the first president to urge NATO allies to boost their share of spending on defense. (Barack Obama famously reproached the allies by saying “we can’t do it alone.”) Yet none of Trump’s predecessors ever really questioned the existence of the alliance itself. Pressuring allies to get their act together is one thing; sowing doubt about the United States’ willingness to defend its partners is something entirely different. And while most Americans are fine with urging allies to do more for their own defense, there’s little evidence that they’re keen to blow up NATO altogether. “If he’s just driving a better bargain, most Americans will be fine with it,” says Cronin. “But I don’t think there’s any fundamental disagreement on the mainstream view that transatlantic allies share the same interests and values — and that it’s better to work together than apart.”

And that raises an even more disturbing question: Why would Trump go to such lengths to undermine NATO if he doesn’t stand to gain voter approval by doing? It could be that he’s acting according to a deep-seated conviction that the United States would be better served by going it alone; he does, after all, have a long-standing record of criticizing our friends overseas. But perhaps it’s also worth noting that Trump’s incendiary comments about NATO are guaranteed to bring a big smile to the face of the man the U.S. president will be meeting in a few days. That would be Vladimir Putin.

Christian Caryl is an editor with The Post’s Opinions section. Follow Christian on Twitter @ccaryl.