NATO had already agreed to establish battalions of 800 to 1,200 multinational troops in Poland, Lithuania, Estonia and Latvia, as well as a brigade of four more battalions to be based in Romania and Bulgaria. The battalion in Poland is to be commanded by the United States and composed almost entirely of American forces.

Mr. Stoltenberg called it “the biggest reinforcement to our collective defense in a generation.”

While the reinforcement was prompted by Russia’s annexation of Crimea in 2014 and by what the Western alliance regards as continuing belligerent behavior from Moscow, NATO leaders insisted the reinforcement did not constitute the advent of a fresh Cold War.

Neither, Mr. Stoltenberg said, has the alliance achieved the closer relationship it wished to forge with Moscow after the collapse of the Soviet Union.

“We are in a new situation, which is different from anything we have experienced before,” he said.

Despite concerns that some NATO nations were softening in their support for sanctions against Russia, Mr. Cameron insisted that the alliance’s message to Russia had not changed.

“We are not seeking confrontation with Russia,” he said. “Indeed, we are working to prevent it. So we will continue to pursue a twin track of deterrence and dialogue.”

Some issues have not changed during Mr. Obama’s presidency. NATO’s leaders agreed to extend its mission in Afghanistan through 2017, after the president’s announcement on Wednesday that the United States would keep 8,400 troops in the country. Mr. Stoltenberg said the alliance was close to reaching its goal of $1 billion to finance the Afghanistan mission.

Chancellor Angela Merkel of Germany made clear that worries about mass migration — one million migrants arrived in her country last year from Afghanistan and elsewhere — had factored into the decision to further support the Afghan government.