Trump pushes in front of Marković at the 2017 NATO summit. Kevin Coombs / Reuters

President Donald Trump seems to have complicated feelings about Montenegro, the former Yugoslav republic whose admission into NATO he approved last year.

At Trump’s first NATO summit in 2017, he shoved aside Montenegro’s prime minister, Duško Marković, during the so-called family photo that brings together the leaders of the alliance’s member states in order to get a more prominent position in the picture. Then, in an interview broadcast Tuesday on Fox News, Trump called Montenegro, a country with roughly the same population as Vermont (about 620,000 people) and about the same geographic size as Connecticut, “a tiny country” with “very aggressive people.”

“They may get aggressive, and, congratulations, you’re in World War III,” he said.

Trump was responding to a question from Tucker Carlson, the Fox News host who asked him why, in the event Montenegro were to be attacked, “ should my son go to Montenegro to defend it from attack?” Trump responded: “I understand what you’re saying. I’ve asked the same question.”

The answer is Article 5 of the Washington Treaty, which established NATO. This part of the treaty declares that an armed attack on one member of the alliance is an attack on all members, and commits each member to collective self-defense. Article 5 has been invoked only once, by the United States following the attacks of 9/11. That’s the reason why, nearly 17 years later, troops from NATO countries are still in Afghanistan, where al-Qaeda planned the 9/11 operation that killed 3,000 people.

Trump has raised similar questions before, but these latest remarks come after a tense meeting with NATO allies and an unusually chummy one with Russian President Vladimir Putin. And though Trump in Brussels signed on to a statement reaffirming Article 5 among other things, he has returned to the United States once again sounding skeptical. And his remarks also underscore familiar questions: How much do U.S. allies need to spend on defense in order to placate Trump’s concerns that they are getting a free ride? Is NATO’s continuing eastward expansion worth irritating Russia over? What does the idea of NATO’s collective self-defense mean?

NATO was established with 12 countries after World War II as a bulwark against the Soviet Union and its Eastern bloc allies. Over the next few decades of the Cold War, the alliance expanded to include four more nations. The U.S. set up bases and put nuclear missiles and hundreds of thousands of troops in the alliance’s member states. When the Cold War ended with the dissolution of the Soviet Union, NATO faced an existential question: What purpose did it serve when its raison d’être, the Soviet threat, was gone? The alliance decided that it would refashion itself for a new era—and it expanded to include states such as the Czech Republic, Hungary, and Poland, which were previously part of the Moscow-allied Warsaw Pact nations, and then, subsequently, more Eastern European states.

On the face of it, this was innocuous. But in Moscow, there was alarm and a deep sense of betrayal. Many Russians say they believe that during the breakup of the Soviet Union, the U.S. promised Moscow that NATO wouldn’t expand eastward. It’s a line Putin has repeated several times. Most Western historians dispute that any such assurance was made, and Mikhail Gorbachev, the last Soviet leader, has also rejected the idea. Still, this difference in perception between NATO and Russia is at the heart of many of the current tensions between the two sides: Every time the alliance has expanded eastward to include states Russia sees as part of its sphere of influence, tensions between the two sides have risen.

Enter Montenegro, which became a NATO member last year. Russia and Montenegro have traditionally been allies—with close cultural and economic ties. So the question of NATO membership became a serious bone of contention between them. In 2016, Montenegro accused Russia of plotting an election-day coup to kill the then-prime minister in order to seat a pro-Moscow politician who would, Montenegrin prosecutors have said, reverse course on NATO membership. A trial of the alleged coup plotters is ongoing. Russia denies any role, though after Montenegro joined NATO last year, Moscow vowed “retaliatory measures” against its former ally’s “hostile course.”

Which brings us back to Trump’s remarks about Montenegro that were broadcast Tuesday. They came soon after his meeting in Helsinki, Finland, with Putin and their now-infamous news conference. The comments also came just days after Trump’s meeting with fellow NATO leaders in Brussels at a conference that was marked by rancor. Trump, as he has done repeatedly, demanded that NATO’s members spend more on defense. He repeated those comments in his Fox interview.

“I went there and said, ‘Tough, you’re going to pay because we’re not going to pay for 70 to 90 … percent—we’re not going to pay 90 percent of the cost to defend Europe,” he told Carlson. (U.S. defense spending accounts for about 72 percent of overall NATO defense spending.)

Trump’s predecessors have also urged NATO’s members to spend more on defense, and those countries have pledged to spend more in recent weeks and months. But even if each NATO country reaches an agreed guideline to spend 2 percent of gross domestic product on defense, the U.S. will remain NATO’s dominant power, as it is by far the world’s largest military spender. This is likely to remain a matter of irritation for Trump, who has repeatedly said that U.S. allies are taking it for a ride.

“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,” Trump told Carlson. “And I said: ‘You have to pay. You have to pay … They are not even paying and we’re protecting them. That’s the equation on Montenegro.”

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