Mr. Putin used historical arguments to claim Crimea. He recently inaugurated a similar discourse on southeastern Ukraine, noting that huge parts of it were called Novorossiya, or New Russia, when first captured in czarist times. The rights of ethnic Russians still living there need to be protected, he said.

Significant Russian military assets, including intercontinental ballistic missiles, Navy ship gears, and jet and helicopter engines, are produced in eastern Ukraine. Vladislav Zubok, a Russian Cold War expert teaching at the London School of Economics who has been researching the 1991 collapse of the Soviet Union, said senior Soviet officials were panicked at the prospect of losing both Crimea and Ukraine’s industrial heartland. So the current crisis has deep roots.

But nothing is that straightforward.

Perhaps a more significant precedent, Professor Zubok said, are the high-profile military maneuvers, without an invasion, long recommended by the K.G.B. to destabilize restive neighbors. Russia deployed that tactic in Berlin in 1958, and in Poland during the 1980-81 Solidarity uprisings, for example. If Moscow is following that strategy now, no invasion is imminent, he said.

The main factor arguing against invasion is the risk to Russia’s prosperity, which Mr. Putin restored.

“Putin will have to explain why he is risking war and sanctions and how he will improve the lot of seven million people there,” Professor Zubok said. “How to do that and still maintain the standard of living of all Russians? He would really be saying: ‘Guys, it is all for the Russian motherland now. It is time to tighten your belts.’ ”

The economic fallout from Crimea has already shoved Russia toward recession, with capital flight and skittish foreign investors. Russia’s credit rating was cut Friday by Standard & Poor’s to just one notch above “junk” status, pushing up the cost of much-needed loans abroad.

Mr. Putin and his closest advisers and allies have brushed off the Western travel and banking sanctions imposed on them after the seizing of Crimea. But the threat of a major economic blockade, sanctions against entire sectors of the economy that would probably be set off by a Ukraine invasion, are another matter.