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LONDON — President Donald Trump has often shocked and confounded the world with his unique brand of rhetoric. But on Thursday it was what he did not say that caused a stir.

The 45th president had been expected to promise that America would defend its NATO allies if they ever came under attack. That principle of collective defense is, in theory, cemented by Article 5 of the alliance's charter, NATO's core tenet. It means that "an attack against one ally is considered as an attack against all."

No other president since NATO was founded in 1949 has questioned that principle — until Trump.

He's called the alliance "obsolete" and has repeatedly urged its members to pay more toward bolstering their own militaries. Many of these nations do not currently meet NATO's recommended spending targets, and Trump has threatened that, unless they up their game, the U.S. might not back them up in a fight.

Asked in a New York Times interview last July whether he would protect smaller states from Russia, he said his support would be conditional on them paying up.

"If they fulfill their obligations to us, the answer is yes," he said.

"Once you start to undermine the alliance then the whole post-Cold War order breaks down"

As he has for many of his policies, Trump has given mixed messages on NATO. In April, he backtracked on the "obsolete" comment and called NATO a "bulwark of international peace and security."

But he has never explicitly endorsed Article 5.

Many commentators expected that to change Thursday when Trump gave a speech at NATO's headquarters in Brussels. After all, he was speaking alongside a mangled girder from the World Trade Center, a shrine whose very name was "The 9/11 and Article 5 Memorial."

The name refers to the attack on New York's twin towers, the only time Article 5 has actually been invoked. More than 1,000 military personnel from America's NATO allies have died in the subsequent U.S.-led war in Afghanistan.

President Donald Trump walks past French President Emmanuel Macron and German Chancellor Angela Merkel during the NATO summit in Brussels, Belgium, on Thursday. Jonathan Ernst / Reuters

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Ahead of Thursday's speech, The New York Times quoted an administration official who was "briefed on the president's planned remarks" and said that Trump would make the promise.

But his speech stopped short of doing so.

Trump thanked other members for their support following 9/11 — "our NATO allies responded swiftly and decisively" — but was far from explicit on Article 5.

The president's spokesman, Sean Spicer, told reporters afterward that Trump's mere attendance was a tacit acknowledgement of his commitment to the mutual-defense clause.

"We all understand that by being part of NATO we have treaty obligations and commitments that we made as being part of NATO," Spicer said. "So to have to reaffirm something by the very nature of being here and speaking at a ceremony about it is almost laughable."

Many experts disagree.

Any sign that the U.S. might blink first could be taken as a signal by Russian President Vladimir Putin that cracks are appearing in NATO, according to many Western analysts.

"Article 5 is the whole point of NATO," said James Nixey, head of the Russia and Eurasia program at London's Chatham House think tank. "Articles 1, 2, 3, and 4 are all about talking. Article 5 is all about action — it's the only article that really means anything."

Although talk of all-out-war between America and Russia may seem remote, the possibility that Moscow may try to extend its influence in Eastern Europe had increased in recent years, according to many Kremlin-watchers.

Putin will be thrilled at Trump's refusal to endorse Article 5. Unimaginable under any other president. — Tom Wright (@thomaswright08) May 25, 2017

Some of the countries that are now in NATO were formerly part of the Soviet Union, a communist bloc controlled by Moscow whose disintegration Putin has called "a major geopolitical disaster of the century."

The Russian president enjoys sky-high domestic popularity — all built on his self-styled image as a man who can restore Russia to its former glory. He sees NATO as a Western encroachment on Russia's borders.

In March, NBC News traveled to Latvia, one of the former Soviet countries now in NATO.

Some people living there, around 20 miles from the Russian border, said they felt Trump's ambivalence toward NATO put them in danger of increased Russian influence. They feared a similar fate as Ukraine, which has been fighting rebels allegedly backed by Russia for the past three years.

Trump is hardly the first president to press NATO allies to spend more.

NATO recommends that each nation spend 2 percent of its gross domestic product on defense. Only five of the 28 members currently do so — the U.S., Greece, Estonia, the U.K. and Poland.

Trump railed against this shortfall Thursday, telling the audience in Brussels that it was "not fair to the people and taxpayers of the United States."

His words echoed those of President Barack Obama, who just last year accused NATO members of being "complacent" and told them to dig deeper into their wallets.

What's different with Trump is that no other president has accompanied this plea with an ultimatum: Pay up or we won't protect you.

Nixey, at Chatham House, agreed that "European states have to pay up more."

But whatever the cost, NATO has always been a trade-off between the U.S. and its smaller allies. Washington protects them and in return gets stability and security along its allied border with Russia and beyond.

"It depends whether you believe that America has a role to play in global security," Nixey said. "If you do, then NATO is critical."

At its heart, the alliance is "all about values," he added. "Most NATO states are committed to democracy. Once you start to undermine the alliance then the whole post-Cold War order breaks down."