Republican presidential nominee Donald Trump during the third day of the Republican National Convention on Wednesday, July 20, 2016. (Jabin Botsford/The Washington Post)

Presidential candidate Donald Trump has set off alarm bells with a suggestion that a Trump administration would not automatically defend fellow members of NATO from a Russian attack if they have not lived up to their financial obligations.

Trump, in an interview transcript published Thursday in the New York Times, went beyond his earlier assertions that he might reconsider the U.S. role as one of 28 nations in the North Atlantic Treaty Organization, because many European countries are not spending enough on defense.

When asked whether he would provide military aid to the Baltic countries if Russia were to attack, Trump replied, “If they fulfill their obligations to us, the answer is yes.”

Trump was more vague when asked what he would do if the answer were no.

“Well, I’m not saying if not,” he said. “I’m saying, right now there are many countries that have not fulfilled their obligations to us.”

The Republican nominee’s remarks provoked a swift rebuke from NATO Secretary General Jens Stoltenberg.

“Solidarity among allies is a key value for NATO,” he said in a statement in which he did not mention Trump by name, saying he will not interfere in U.S. elections. “This is good for European security and good for U.S. security. We defend one another.”

Noting that the United States has always stood by Europe since NATO was formed as the bedrock of security after World War II, Stoltenberg added, “Two world wars have shown that peace in Europe is also important for the security of the United States.”

Though U.S. administrations have for decades complained that Europe is getting a free ride, Trump’s comments represented a repudiation of Article 5, the heart of the alliance, that an attack on one is considered an attack on all. Article 5 was first invoked after the terrorist attacks against the United States on Sept. 11, 2001, and is the reason European and Canadian troops were sent to Afghanistan.

“There’s no question the NATO allies could be doing more,” said James Goldgeier, dean of the School of International Service at American University and a fellow at the Council on Foreign Relations.

“But nobody ever said we’re going to pick and choose who we’re going to defend based on how much they’re spending. The alliance would unravel if everyone did that.”

For some in the Baltic states — Estonia, Latvia and Lithuania — the American businessman’s comments provoked confusion and surprise.

Estonian President Toomas Hendrik Ilves pointed to his country’s role in the war in Afghanistan as proof of the country’s commitment, sharing on Twitter a message that said Estonia had one of the highest casualty rates per capita in the conflict. “Estonia’s commitment to our NATO obligations is beyond doubt and so should be the commitments by others,” the Estonian Foreign Ministry added in an emailed statement.

“We take our commitments seriously,” Latvian Foreign Minister Edgars Rinkevics said in Washington, where he was attending meetings to coordinate military action against the Islamic State. “We hope and expect that all our allies, big and small, take their commitments the same.”

It was not the first time that Trump has criticized NATO, formed in 1949 by the United States, Canada and 10 European countries to defend one another against the Soviet Union. His primary objection has been economic. Washington pays about a fifth of NATO’s direct costs, more than any other country, and roughly 75 percent of all military spending, according to a 2015 NATO report.

In an interview with The Washington Post editorial board published March 21, Trump called NATO a “good thing to have” but said it was obsolete and no longer affordable in an era of large U.S. deficits.

“I don’t want to pull it out,” he said of U.S. membership, adding: “NATO was set up when we were a richer country. We’re not a rich country. . . . NATO is costing us a fortune and yes, we’re protecting Europe but we’re spending a lot of money. Number one, I think the distribution of costs has to be changed. I think NATO as a concept is good, but it is not as good as it was when it first evolved.

He made a similar argument at a CNN town hall.

“Frankly, they have to put up more money,” he said. “We are paying disproportionately. It’s too much, and frankly it’s a different world than it was when we originally conceived of the idea.”

Military spending by NATO allies has long been a bone of contention for U.S. officials. NATO members are expected to spend 2 percent of their gross domestic product on defense. In fact, only five countries do, among them the United States.

In a 2011 speech when he was defense secretary, Robert M. Gates warned that NATO faced “military irrelevance” if Europeans did not spend more.

“The blunt reality,” Gates said, “is that there will be dwindling appetite and patience in the U.S. Congress — and in the American body politic writ large — to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defense.”

But no critics have gone as far as Trump did in the Times interview, representing a change from decades of internationalist Republican foreign policy and a rejection of the idea of America as the “indispensable nation.”

“This is as opposite to the Reagan approach to foreign policy as you could get,” Goldgeier said.

“You can argue the Europeans and the E.U. should do more to defend themselves. But you have to have a shared commitment. It’s not foreign policy going to the highest bidder.”

Two weeks ago at a NATO summit in Warsaw, President Obama reaffirmed that the United States has Europe’s back, saying that “in good times and in bad, Europe can count on the United States — always.”

White House press secretary Josh Earnest said Thursday that the U.S. commitment to mutual defense is “ironclad.”

“The recognition that NATO is the cornerstone of American national security is a policy that the United States has pursued under every post-World War II president, Democratic and Republican,” he said.

But Earnest acknowledged that Obama believes Europe needs to pay its “fair share.” He noted that NATO member countries have agreed to spend 2 percent of their GDP on defense by 2024.

Sean Kay, who chairs the international studies program at Ohio Wesleyan University and informally advised Sen. Bernie Sanders (Vt.) during his run for the Democratic presidential nomination, called Trump’s approach “more like a bull in a china shop.”

“It’s in a long tradition, from the founding of NATO, to get the Europeans to defend themselves and be less reliant on the United States,” he said. “The problem is the way he’s going about it. It does damage to the idea.”

Watch the White House response at 8