Eighteen months into his presidency, Donald Trump continues to display near-complete ignorance about how NATO works and why the alliance matters. He tweets that the United States gives 4 percent of its GDP to NATO. (Not so: We spend 4 percent on defense; we actually contribute less than $500 million directly to NATO for its combined civil and military budgets, or less than one-10th of 1 percent of our defense budget.)  He sees NATO as a charity project for Europeans rather than a cornerstone of U.S. national security. And, as we learned from his Fox News interview with Tucker Carlson on Tuesday night, he is willing to call into question Article 5 — the alliance’s mutual defense provision. He appears either unable to comprehend that an ironclad commitment to Article 5 is at the heart of NATO, or is purposefully undermining the alliance in the wake of his traitorous performance in Helsinki.

In his interview, Carlson didn’t evoke Article 5 with respect to Germany or France. He chose to pick on tiny Montenegro. Despite Montenegro’s having joined NATO only last year, during Trump’s presidency, Trump readily agreed. He bizarrely characterized Montenegrins as “aggressive” and suggested that our commitment to defend them might ensnare us in “World War III.”

Most commentators seized on the fact that by publicly questioning Article 5, which has been invoked only once — in defense of the United States after the 9/11 attacks — Trump had done further damage to NATO and, no doubt, elicited more glee in the Kremlin. But Carlson and Trump’s example was also curious because Montenegro is exactly where Putin himself might start if he were seeking to undermine NATO solidarity. In 2008, Putin told a surprised George W. Bush that “Ukraine is not a country”; during their one-on-one meeting on Monday, did Putin tell Trump — in one way or another — that “aggressive” Montenegro shouldn’t be in NATO?

In the months preceding NATO’s invitation to Montenegro to begin the accession process in December 2015, my Russian counterparts in Vienna at the Organization for Security and Co-operation in Europe, where I was the U.S. ambassador, grew increasingly vocal. They never talked about Montenegro’s joining NATO; instead they expounded with dramatic flourish about political divisions and unrest, painting a picture of a dysfunctional and troubled country, trying to turn NATO members against the idea. I understood what they were doing, but I remember thinking: “It’s amazing how much the Russians care about Montenegro joining NATO.”

Montenegro is a jewel of a country on the Adriatic, one of seven countries that ultimately emerged from the collapse of the former Yugoslavia. It is a young democracy and, like others, continues to fight corruption. While it does have a coast, it’s hard to imagine a port in Montenegro posing a direct challenge to Russia. Its small population of around 620,000 includes a military of just under 2,000 people with a budget well under $100 million. It is good to have Montenegro in the NATO family — and it has contributed troops to NATO’s missions in Afghanistan for years, even before it was a member — but Montenegro does not dramatically change the total military capability of the alliance.

But NATO did dramatically enhance the independence and prospects of Montenegro. On the day NATO’s invitation was announced, my Montenegrin colleague in Vienna was grinning widely.  She had helped negotiate Montenegro’s independence a decade earlier, and as she hugged me she declared, “It’s as if a weight that has been on my shoulders forever, for my whole life, has been lifted.”

Then I realized: The reason the Russians cared so much about blocking NATO membership is that they knew it would reduce their ability to manipulate Montenegro and would remove a chess piece from a board on which they sought to challenge Europe. Indeed, in the months after NATO’s invitation, Russia continued its attempts to stymie Montenegro’s path by stirring up internal unrest through propaganda, paid protesters and even an attempted coup. They failed.

Today, Russia is playing a similar game in the country soon to be called North Macedonia. A long-standing dispute between Skopje, the (North) Macedonian capital, and Athens over the name of the small country to Greece’s north has finally been resolved, and thus a major obstacle to Skopje’s accession to NATO has been removed. Cue the Russians! Greece expelled two Russian diplomats earlier this month because they were trying to stir up opposition to the name deal — the Russians want to restore obstacles to (North) Macedonia’s progress.

This year marks the centenary of the end of World War I. The spark that ignited that war was an assassination in Sarajevo, less than 200 miles from Montenegro’s capital, Podgorica. Perhaps we should give Trump the benefit of the doubt and assume he was drawing a lesson from history. But if the lesson he took was that we shouldn’t concern ourselves with little countries else we risk large entanglements, then the lesson was the wrong one.

After two world wars a generation apart, the United States and its allies have built — imperfectly and unevenly, to be sure, but also impressively — an international system encoded in law and grounded in universal principles. NATO has been critical to defending the part of the world in which those rules are most firmly established and has helped create the space for a growing area of stability and prosperity. The world we inhabit today is different from the world we inhabited in 1914, and NATO is a cornerstone of our present advantage.

Little countries may be where menaces look to begin their mischief, but that’s not where they end it. Putin doesn’t want to be able to destabilize or manipulate Montenegro because of his policy toward Montenegro; he wants to use Montenegro as a pawn in a larger game.  He wants southeast Europe as a place where he can foment unrest, dial up tensions and tie up European attention and resources. And he wants to undermine the rules of the international system by showing we lack the will to enforce them. Putin seeks to test the international community’s resolve not just once, but again and again, each time in a larger way. Russia violated the sovereignty of Moldova, then Georgia, then Ukraine. Perhaps a NATO country is the next target.

By maintaining our steadfast commitment to NATO, including to its smallest members, we reinforce the alliance’s credibility and the international system we and our allies have built. NATO’s protection makes possible a non-zero-sum mode of international politics and at the same time deters those who wish to return to a 19th-century balance-of-power approach and the wars that inevitably attend it.

If we were to give up on Montenegro (or the Baltics or any NATO member), we would give up on NATO itself, and hasten a return to the kind of world in which two world wars emerged. The president is wrong: World War III won’t come from our standing by our pledge to Montenegro and other NATO allies, it will come from our abandoning it. Trump’s behavior in Helsinki, his kowtowing to Putin, his calling into question our NATO commitment — all this makes us, and the world, less safe.