LONDON — In the 12 short years since Montenegro regained its independence, the European country has joined NATO, boosted its defense spending, and according to official figures contributes more troops per capita to the war in Afghanistan than the United States.

Yet this U.S. ally — smaller than Connecticut and about as populous as Baltimore — found itself in President Donald Trump's crosshairs late Tuesday as he once again criticized NATO.

The president suggested he would be unhappy defending "tiny" Montenegro if it were attacked, calling into question NATO's central principle of mutual defense.

He also questioned whether the country's "very aggressive people" could draw NATO into a war with Russia.

Like other presidents before him, Trump wants smaller NATO members to pull their weight by spending more on their militaries. But he's the first to directly challenge the alliance's mutual defense clause. Critics say that destabilizes one of the foundations of the post-World War II Western world.

Trump did not bring up Montenegro himself. It was used as an example by Tucker Carlson during an interview on Fox News.

"Membership in NATO obligates the members to defend any other member that's attacked," Carlson said. "So let’s say Montenegro, which joined last year, is attacked. Why should my son go to Montenegro to defend it from attack?"

Military helicopters stand on the tarmac at Golubovci airport, near Podgorica, Montenegro. Darko Vojinovic / AP file

Trump answered: "I understand what you're saying. I've asked the same question. Montenegro is a tiny country with very strong people. … They're very aggressive people. They may get aggressive, and congratulations you're in World War III."

Fourteen people are currently on trial in Montenegro accused of plotting to kill the prime minister and stage a coup to bring a pro-Russian party to power. The government says one of the aims of the plan was to stop the country from joining NATO. The Kremlin has denied any link to the alleged plot.

By saying that he had "asked the same question" why Americans troops should be asked to defend their allies, Trump was challenging the entire point of NATO.

Article 5 of its founding treaty states that an attack on one member "shall be considered an attack against them all" and is at the "very heart" of the organization. It doesn't specifically mandate allies respond with force but that's the spirit in which many see it.

The only time Article 5 was invoked was on behalf of the U.S. after the 9/11 attacks in 2001.

It's not the first time Trump has questioned the principle of mutual defense. During his presidential campaign he told The New York Times that his willingness to protect smaller states from Russia was conditional on their defense spending. He was also slow to commit to Article 5 once in office.

In his Fox News interview Tuesday, Trump suggested again that he did not like the idea of mutual defense, saying of NATO "but that's the way it was set up."

"Don't forget, I just got here a little more than a year and a half ago, but I took over the conversation three or four days ago and I said you have to pay," he said.

This was in reference to NATO's summit last week in which Trump caused panic across Europe and in the Pentagon itself after threatening to reassess Washington's defense commitments.

Trump's main contention with NATO is that smaller members expect to be defended by the U.S. but don't want to share their burden financially.

While the NATO meeting in Brussels was an acknowledged triumph, with billions of dollars more being put up by member countries at a faster pace, the meeting with Russia may prove to be, in the long run, an even greater success. Many positive things will come out of that meeting.. — Donald J. Trump (@realDonaldTrump) July 18, 2018

Other presidents including Barack Obama have argued the same point, and many experts agree to the principle that other countries should pay more.

What Trump's critics object to is the manner in which he has gone about it, issuing harsh rhetoric that destabilizes the organization, and criticizing and undermining allies while appearing softer on Russian President Vladimir Putin.

He has also repeatedly mischaracterized how NATO spending works as well as the extent that the U.S. picks up the tab for its allies.

In 2014, every member of the alliance agreed that by 2024 they would each spend 2 percent of their national wealth on defense. Currently, only five nations achieve this, and Trump has accused certain nations, such as Germany, of lagging behind.

Montenegro spends around 1.5 percent of its national budget on defense, according to NATO statistics, and it says it will hit 2 percent by the target date of 2024.

It currently contributes 20 troops to the NATO operation in Afghanistan, which, with Montenegro's population of around 620,000, is more per capita than the U.S. official contribution of 8,475 (although Pentagon officials say the true number is higher).

Trump has a history with the small Balkan nation.

At last year's NATO summit, the president pushed aside Montenegro Prime Minister Duško Marković as the leaders prepared for a group photo.