As Russia has grown more assertive in recent years, in part to compensate for its weakness relative to the West, much of the friction between it and NATO has played out along the Russian-Baltic borders.

The dynamics in the region have been governed by the alliance’s pledge to defend the Baltic nations if they are attacked, a threat meant to be so serious that it prevents war from ever occurring. That pledge is an important pillar of stability in a region where, in a highly unlikely but plausible worst case, a conflict could escalate to war between nuclear powers.

Mr. Trump’s comments suggested that under his leadership, that guarantee would no longer be guaranteed — that the pillar might not stand if he believed the Baltic nations had not “fulfilled their obligations to us.”

How realistic is the hypothetical?

The premise of the question posed to Mr. Trump, a Russian attack on the Baltics, might seem outlandish. And indeed, no one seriously thinks that Russian tanks are going to pour into Latvia any time soon.

Rather, there are two scenarios that worry analysts and policy makers. One is a Russian provocation meant to test NATO’s promise to the Baltics, with the aim of unraveling that commitment while avoiding outright war. The other is that some unforeseen accident or miscalculation along the border, if it came at a time of high tension, could be misconstrued as an act of war, setting off a rapid mutual escalation that could lead to unwanted conflict.

The odds of either scenario are low, but the stakes are potentially catastrophic. The first could erode or outright end the European defense unity that grew out of the ashes of World War II, and would leave Eastern Europe once again vulnerable to Russia. The second could mean war among nuclear powers.