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.