LONDON — Russia’s annexation of Crimea has suddenly revived the North Atlantic Treaty Organization’s central role as a counterweight to Moscow, and with it questions about the alliance’s options and ability to act.

Vice President Joseph R. Biden Jr. swept into Poland and the Baltic nations on Tuesday with a message of reassurance that their membership in NATO carries the protection of the United States. But given deep Western reluctance to use military force in response to Russia’s aggression, it remains unclear what the alliance’s commitment to collective security means for Ukraine and other nonmembers should President Vladimir V. Putin continue to try to expand Moscow’s influence in the former Soviet bloc.

Ian Bond, the director of foreign policy at the Center for European Reform, a London-based research group, said that “Putin has just given NATO something to do, but the question is whether NATO is up to it.” It is now crucial to deter further moves by Mr. Putin, he said. “If Russian forces move into eastern Ukraine,” he said, “what would NATO do?”

The Atlantic alliance was designed, as the old phrase went, to keep the Americans in, the Germans down and the Soviets out. Now, with Mr. Putin acting more like a Cold War antagonist, arguing that Russia has the right to defend Russians everywhere, the United States will be under more pressure to sustain and exhibit military strength in Europe despite the much ballyhooed “pivot to Asia.”