On his stops in Europe, Biden adds to talk of a new Cold War.

President Biden and his national security team have contended since he took office that all the easy, tempting comparisons between this era and the Cold War are misleading, a vast oversimplification of a complex geopolitical moment.

The differences are, indeed, stark: The United States never had the kind of technological and financial interdependence with its Cold War adversary, the Soviet Union, that so complicates the increasingly bitter and dangerous downward spiral in the relationship with China.

And Mr. Biden’s advisers often argue that Russia is not the Soviet Union. Yes, it has nuclear weapons, they say, but its conventional military capacity has now been severely degraded in Ukraine.

And in Soviet times, the United States felt compelled to fight an ideological battle around the world. In the new era, it is fighting China’s efforts to use its economic and technological power to spread its influence.

Nonetheless, the echoes of the Cold War are growing louder. Mr. Biden himself added to the din this week. In Vilnius, Lithuania, on Wednesday night, addressing a crowd that was waving American, Lithuanian and Ukrainian flags, he repeatedly invoked the struggle of the Baltic nations to free themselves from a collapsing Soviet Union, and told Vladimir V. Putin that the United States and its allies would defend Ukraine, and with it other vulnerable parts of Europe, “as long as it takes.”

Mr. Biden never quite said explicitly that the United States must again “bear the burden of a long, twilight struggle” — President Kennedy’s famous description of the Cold War in his 1961 inaugural address, as it entered its most dangerous phase. But Mr. Biden’s message was essentially the same.

“Our commitment to Ukraine will not weaken,” he said. “We will stand for liberty and freedom today, tomorrow, and for as long as it takes.”