WASHINGTON — Just hours after President Vladimir V. Putin of Russia boasted last week about producing “invincible” new nuclear arms designed to evade American missile defenses, President Trump’s nominee to command the nation’s military cyberunits was being grilled at a Senate confirmation hearing about another vexing Russian threat.

Lt. Gen. Paul Nakasone, the nominee to run the National Security Agency and United States Cyber Command, acknowledged that plans were in place to strike back at Moscow for its election hacking. Those actions would require Mr. Trump’s approval.

But General Nakasone said the Russians, as well as America’s other adversaries in cyberspace, seem unimpressed.

“I would say right now they do not think much will happen to them,” he said. “They don’t fear us.”

The Russian muscle-flexing and the American hand-wringing captured a strategic vacuum that now envelops Washington as Mr. Putin pursues what he views as a complementary new-generation nuclear arsenal, as well as cyberweapons.