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The immediate concern is that a heightened alert level, by design, loosens the safeguards on nuclear weapons, making it more possible that they could be used, by accident or design.

In recent years, Russia has adopted a doctrine that lowers the threshold for using nuclear arms, and for making public threats of unleashing their powers in deadly atomic strikes.

“It’s what he does,” Hans M. Kristensen, the director of the Nuclear Information Project at the Federation of American Scientists, a global policy think tank in Washington, said in an interview. “It’s verbal saber-rattling. We’ll see where he goes with it. This war is four days old and he’s already made nuclear threats twice.”

Mr. Kristensen noted that in 2014, when Mr. Putin annexed Crimea, the peninsular part of southern Ukraine that juts into the Black Sea, the Russian president also raised the possibility that his forces might turn to atomic weapons. He recalled that when Mr. Putin was asked how he would react to retaliatory sanctions by the West, he “said he was willing to put his nuclear forces on alert.”

Mr. Putin’s announcement on Sunday came hours after Europe and the United States announced new sanctions, including banning some Russian banks from using the SWIFT financial messaging system, which settles international accounts, and crippling the Russian central bank’s ability to stabilize a falling ruble.

Matthew Kroenig, a professor of government and foreign service at Georgetown University who specializes in atomic strategy, said history bristled with cases in which nuclear powers had threatened to unleash their arsenals on one another. He pointed to the Berlin crisis of the late 1950s, the Cuban missile crisis of 1962, a border war between the Soviet Union and China in 1969, the Arab-Israeli war of 1973, and a war between India and Pakistan in 1999.

He also noted that Mr. Trump had leveled similar threats against Kim Jong-un, the North Korean leader, after his armed forces conducted a series of long-range missile tests. In his first year in office, 2017, Mr. Trump threatened “fire and fury like the world has never seen.”