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.
"The scenario is like what happened to Iran," said Igor Korotchenko, a member of a civilian board that advises the Russian military and the editor in chief of National Defense Magazine. "The Russian Federation is not interested in bringing troops into eastern Ukraine."
European firms seek to minimize Russia sanctions
Beyond economics, the specter of Slavs killing Slavs would soon sour the Russian public on any invasion. Although the Ukrainian Army is weak, it numbers 70,000, and the country has a history of partisans' attacking invaders. Mr. Korotchenko said Russians would probably embrace military intervention only if the army was dispatched as a peacekeeping force should the Ukrainian military cause mass casualties.
And the locals could also prove hostile. In eastern Ukraine, regional polls have found that at most one-third of the population, depending on the city, supports joining Russia.
In Crimea, Russian soldiers were greeted warmly, and needed to hold only the Isthmus of Perekop, three miles wide, to sever Crimea from Ukraine. In eastern Ukraine, if the 40,000 Russian troops now estimated to be camped along the border crossed over, they would probably be attacked. Russia would also be responsible for a flood of refugees.
"You cannot occupy this region only with these small green men," said Alexander M. Golts, an independent Russian military analyst, referring to the anonymous soldiers in Crimea whom Mr. Putin later admitted were elite Russian soldiers. "So you beat those poor Ukrainians. What then? You will have to establish a new border. You will not need 40,000 troops, you will need 140,000."
Ultimately, analysts said, it is much more advantageous — and far cheaper — for Russia to manipulate a low-grade mutiny with occasional flare-ups.
That will achieve the goal Mr. Putin wants: keeping Ukraine just destabilized enough that it remains an unattractive partner to the European Union or NATO. Russia played out the same script before in Georgia and Moldova.
"It would be a tank-free invasion," said Cliff Kupchan, an analyst at the Eurasia Group in Washington. "That is his long game. I think he will try that before he invades."