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LONDON — Donald Trump set off alarm bells in European capitals Thursday after suggesting he might not honor the core tenet of the NATO military alliance.

Trump said the U.S. would not necessarily defend new NATO members in the Baltics in the event of Russian attack if he were elected to the White House.

He told The New York Times in an interview published Thursday that doing so would depend on whether those countries had "fulfilled their obligations to us" in terms of their financial contributions to the alliance.

“You can’t forget the bills," Trump told the paper. "They have an obligation to make payments. Many NATO nations are not making payments, are not making what they’re supposed to make. That’s a big thing. You can’t say forget that.”

Trump's comments were perceived by some analysts as carte blanche for Russia to intimidate NATO allies and a potential harbinger of the alliance's collapse were Trump to be elected.

I have no idea how to convey the enormity of Trump's NATO comments to readers. They literally make World War III more likely. — Zack Beauchamp (@zackbeauchamp) July 21, 2016

Given Trump's views on NATO, and on America's promises to allies, his election would trigger an immediate global crisis. — Jeffrey Goldberg (@JeffreyGoldberg) July 21, 2016

We have had decades of bipartisan commitment to NATO, which has made it the greatest alliance in history. Trump is now threatening that. — Michael McFaul (@McFaul) July 21, 2016

NATO's treaty states that an attack on one member state constitutes an attack on all, a principle enshrined in Article 5 of the alliance's treaty.

"If Trump wants to put conditions through Article 5, he would endanger the whole alliance," said Beyza Unal, a fellow at the London-based Chatham House think tank.

Sarah Lain, a fellow at the Royal United Services Institute, agreed. She said that Article 5 is the "core" of NATO's defense strategy.

"The suggestion that Trump may consider abandoning a guarantee of protection to fellow NATO countries would in some ways indeed make NATO obsolete," Lain told NBC News in an email.

Estonia's president was among the first to hit back, saying on Twitter that his nation has met its commitments and fought — "with no caveats" — when Article 5 was previously used.

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We are equally committed to a l l our NATO allies, regardless of who they may be. That's what makes them allies. — toomas hendrik ilves (@IlvesToomas) July 21, 2016

Article 5 has only been invoked once — in wake of the 9/11 terror attacks on the U.S.

The head of NATO follows later, saying in a statement to NBC News: "I will not interfere in the U.S. election campaign, but what I can do is say what matters for NATO."

"Solidarity among Allies is a key value for NATO," Secretary General Jens Stoltenberg said. "Two world wars have shown that peace in Europe is also important for the security of the United States."

Hillary Clinton's campaign noted that after Trump's remarks, issuing a scathing statement saying it is "fair to assume" that Russian President Vladimir Putin is "rooting" for the Republican candidate.

Jake Sullivan, a senior Clinton policy adviser, noted how the U.S. has for decades given an "ironclad guarantee" to its NATO allies to come to their defense if attacked, "just as they came to our defense" after 9/11.

"Donald Trump was asked if he would honor that guarantee. He said ... maybe, maybe not," Sullivan said in a statement. "The president is supposed to be the leader of the free world. Donald Trump apparently doesn't even believe in the free world."

It's important to remember that Article 5 doesn't explicitly define what allies must do in response to any attack on a member, noted former U.S. Ambassador to NATO Robert Hunter.

"But for the credibility of the alliance and of the United States, there has to be no doubt in the aggressor’s mind that we — the U.S. — would respond," Hunter said in an email to NBC News.

Former NATO Supreme Allied Commander James Stavridis tore into Trump's latest remarks in a new piece in Foreign Policy, writing that "our nation is built on the values that we share with our European colleagues."

"And, by the way, virtually all of them came from Europe’s Age of Enlightenment: democracy, liberty, freedom of speech, freedom of religion, and on and on," he wrote. "Do we really want to replace them with a balance sheet and a 'what have you done for me lately' attitude?"

Reflecting the oftentimes contradictory messages from the campaign, Trump’s campaign chair Paul Manafort later told reporters that the candidate “believes in the [NATO] agreements” and said The New York Times often is “inaccurate.” The newspaper published a transcript of the interview online.

Amidst the uproar over Trump's remarks, the White House on Thursday stressed that America's commitment to NATO’s principle of mutual self-defense was “ironclad.”

“There should be no mistake or miscalculation made about this country’s commitment to the trans-Atlantic alliance,” spokesman Josh Earnest told reporters.

The interview was not the first time Trump raised questions about America's commitment to NATO, but the remarks published on the eve of his address to the Republican convention took on greater weight now that he's formally won the GOP nomination.

Related: Who's Advising Donald Trump on Foreign Policy?

They were the latest illumination of his "America First" approach — which assesses foreign policy in terms of economic benefit and claws back the longstanding role of the U.S. as global watchdog.

“We are going to take care of this country first ... before we worry about everyone else in the world,” he told the newspaper.

When asked about the importance of U.S. global leadership in wake of World War II, Trump interjected "How is it helping us?" He also pointed to trade deficits.

The Republican candidate has been hounded throughout his campaign by criticism over his lack of foreign policy credentials — and unorthodox suggestions.

When asked by The New York Times to elaborate on his plans to defeat ISIS, Trump was reticent.

"I don’t want to be specific because I don’t want ISIS to know what I’m planning," he told the newspaper.

Trump also was asked about Turkey's crackdown in wake of a failed military coup there — and the candidate took another opportunity to drill down on his message to focus on America.

The U.S. isn't "a very good messenger" to discuss civil liberties at the moment, he told the newspaper.

“How are we going to lecture when people are shooting policemen in cold blood?” he said.