Ukraine becomes more of a tinderbox by the day.
Thousands of Russian troops are maneuvering along the border, with Russian fighter jets entering Ukraine's airspace. Ukrainian leaders have warned that border crossings by any soldiers would be considered an invasion, even while the country pursues military operations against a pro-Russian rebellion in the east. Washington and Moscow hurl ever more heated pronouncements. The first casualties lie in fresh graves.
No less an authority than Gen. Philip M. Breedlove of the United States Air Force, NATO's supreme allied commander in Europe, has said Russia could overrun eastern Ukraine in three to five days. In other words, Russia could basically achieve its goal of creating a neutral, weak Ukraine almost instantly.
But will it?
At first glance, the Russian president, Vladimir V. Putin, seems to have strong reasons to dispatch his tanks: shaping the Ukraine he wants well before elections scheduled for May 25 put a new, legitimate government in place; reclaiming an area that was historically part of Russia; gaining direct access to natural resources and factories that have been crucial to Moscow's military-industrial complex since Soviet times. And his land grab of Crimea in March made him wildly popular at home.
Yet the reasons for Mr. Putin to refrain from further military adventurism make a longer, more tangled list: the cost of a huge occupation force and the responsibility for the welfare of millions more people; the effect of new, more severe Western sanctions on an already weak economy; the possibility of significant Russian casualties caused by an insurgency in eastern Ukraine; a new, implacably anti-Russian western section of Ukraine; and likely pariah status internationally.
On balance, the negatives would seem to outweigh the positives, analysts said.
"Military intervention from Putin's point of view is Plan B," Mark Galeotti, a New York University professor and expert on Russia's security forces currently doing research here, said recently. "It is not off the table, but it is not the ideal outcome."
Mr. Putin would rather feed the insurrection from afar, analysts said, never quite allowing the calm that would give Ukraine the opening needed to join the European Union, or worse, NATO. It is a tactic Russia has used successfully in previous attempts by former Soviet republics to shift westward.
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However, any conversation or briefing paper about Russia's next moves begins with a broad caveat. Few expected that Mr. Putin would seize Crimea in a matter of weeks.
"Nobody, including Putin, knows what he may do next as the situation changes," the Royal United Services Institute, a military and security research organization in London, said in an analysis this month.
There are signs that Russia seems poised to invade.
On Thursday, Ukraine started tentative armed operations to dislodge pro-Russian militants from government buildings in 10 eastern towns. Russia countered with extensive military maneuvers along the frontier, including what the Pentagon said were half a dozen violations of Ukrainian airspace in 24 hours. Russia denied that.
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.
"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."
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."