The day after Russian tanks and troops poured across the Ukrainian border on Feb. 24, NATO leaders received a deeply frightening message. The alliance’s secretary-general, Jens Stoltenberg, opened an emergency video summit by warning that President Vladimir Putin had "shattered peace in Europe” and that from now on, he would openly contest the continent’s security order.

However unlikely, Stoltenberg told the leaders, it was no longer unthinkable that Putin would attack a NATO member. Such a move would trigger the collective defense clause in the North Atlantic Treaty, opening the door to the ultimate nightmare scenario: a direct military conflict with Russia.

President Joe Biden, who had dialed in from the White House Situation Room, spoke up swiftly. Article 5 was "sacrosanct,” he said, referring to the "one for all, all for one” principle that has anchored NATO since its founding after World War II. Biden urged allied leaders to step up and send reinforcements to Europe’s eastern flank, according to multiple officials briefed on the call.