HILLARY CLINTON this week told the American Legion that the United States has a “unique and unparalleled ability to be a force for peace and progress, a champion for freedom and opportunity,” and is therefore not just “an exceptional nation” but “the indispensable nation.” For most of the past century — indeed, for most of U.S. history — those would have been unremarkable claims, political boilerplate for a presidential candidate. This year, they make Ms. Clinton herself exceptional.

As the Democratic nominee was quick to point out, Donald Trump has repeatedly objected to the idea of America as exceptional. The term doesn’t apply, he says, when countries such as Germany are “eating our lunch”; for that matter, he said in 2013, Vladi­mir Putin had a point in rejecting the idea. “I can see that being very insulting to the world,” said a man who normally doesn’t shy from giving offense.

Mr. Trump’s position puts him at odds with virtually every Republican president going back to Abraham Lincoln, not to mention every prominent member of the GOP’s present foreign policy establishment. But he speaks for what seems to be a growing part of a citizenry disillusioned with foreign adventures. Pew Research Center surveys show that public support for the idea that the United States “stands above all other countries” has dropped by about a quarter since 2011 and is now a minority view.

President Obama has had his own difficulties coming to terms with American exceptionalism. Much like Mr. Trump, he observed in 2009 that the British and Greeks also think of themselves as exceptional. Pilloried by Mitt Romney, among others, for that diffidence, he eventually embraced the term and, in a speech marking the 50th anniversary of the Selma march last year, found a grounds for it. America was exceptional, he said, because of its ability to face its own demons and change for the better.

Ms. Clinton doubtless does not disagree with that, but her understanding of the American mission is broader; it’s not enough to offer a moral example. “Our power,” she said in the American Legion speech, “comes with a responsibility to lead . . . because when America fails to lead, we leave a vacuum that either causes chaos or other countries or networks rush in to fill the void.” She did not mention Syria, where Mr. Obama rejected her advice to act in 2012, before the rise of the Islamic State, the employment by the Bashar al-Assad regime of chemical weapons against civilians, the destructive intervention by Mr. Putin’s Russia, and the destabilizing flow of more than 1 million refugees to Europe. But that catastrophe offers compelling support for her point.

Ms. Clinton would not, as president, rush to dispatch U.S. soldiers to Syria, or anywhere else. She said a “bedrock principle” was that “we must only send our troops into harm’s way as a last resort, not a first choice.” But, she added, “we must be able to act decisively on our own when we need to.” Again, a sentiment that all modern presidents have embraced, with the partial exception of Mr. Obama, who said such action was possible only in a very narrow set of circumstances. And who knows about Mr. Trump? A president who regards himself, but not his country, as exceptional could be expected to be reliable only in the defense of his own self-interest.