The fall of Ukrainian President Viktor Yanukovych was swift and sudden, culminating in a series of front-page events on Friday and Saturday. The end result is that Ukraine's government is now sliding into Western Europe's orbit and away from Russia. That's a problem for Russian President Vladimir Putin, who, like many Russians, considers Ukraine an integral part of Russia's sphere of influence, with a shared history, culture, and, in many parts of Ukraine, language.

The fear that Putin will send troops into Ukraine — as he did in Georgia in 2008, ostensibly to protect ethnic Russians — is "so palpable," says Steven Lee Myers in The New York Times, that U.S. National Security Adviser Susan Rice warned Russia that responding with force "would be a grave mistake."

First, here's a brief recap of how we got here. (The New York Times and Wall Street Journal both have more thorough narratives.) After the bloodshed in Kiev on Wednesday and Thursday, Yanukovych's support in parliament and among Ukraine's moneyed elite started dissolving. On Friday, he and opposition leaders signed an agreement brokered by European diplomats in which Yanukovych would give up some powers and resign in December. Russia's envoy, Vladimir Lukin, refused to sign the pact.

It didn't matter: The protesters in Independence Square (Maidan) were having none of the agreement. The "self-defense" forces that formed to guard the Maidan had already worked out an agreement with Interior Ministry officials and individual police commanders that the riot police would be given safe passage if they agreed to leave Kiev; thanks to a raid on a government armory in the western city of Lviv, the protesters had access to weapons.

Parliament had passed a resolution Thursday night ordering the riot police to stand down, and by the time Yanukovych and the opposition leaders signed their peace deal on Friday afternoon, the police were leaving town — and leaving the president's office and other key government buildings unguarded. About 4 am on Saturday, Yanukovych and his family left his opulent presidential estate in helicopters, much of his possessions moved out on trucks.

On Saturday morning, opposition militias took control of government buildings and protesters poured into Yanukovych's estate, gawking at his zoo, garage full of vintage cars (and a hovercraft), and well-appointed mansion. On Saturday afternoon, parliament voted to dismiss Yanukovych but he appeared on TV from somewhere in eastern Ukraine to insist he hadn't resigned and was ousted in a coup. Russia is calling it a coup, too.

On Sunday, parliament dismissed the rest of Yanukovych's cabinet, gave interim presidential authority to the newly appointed speaker, Oleksandr V. Turchynov, and put other lawmakers in charge of key ministries. Yanukovych's own party denounced him as a coward and a traitor who spilled Ukrainian blood and bankrupted the country.

On Sunday evening, Turchynov went on national television and vowed to set up a "government of the people," hopefully by Tuesday, and cast the uprising as a pro-Europe referendum. "We have to return to the family of European countries," he said. "We are ready for a dialogue with Russia... on a new, fair, equal, and neighborly basis, acknowledging and taking into account Ukraine's European choice."

On Monday, the acting interior minister issued a warrant for Yanukovych's arrest, accusing the former president of killing civilians.

This sets up some tough choices for Vladimir Putin, says Walter Russell Mead at The American Interest. On the one hand, Putin can't win over the western part of Ukraine without sending in the troops, and "one wonders whether the Putin government could survive a catastrophically expensive war and the ensuing isolation." He would have to act quickly or risk bankrupting his own country — but then again, Putin "likes to move swiftly and present his adversaries with facts on the ground." Mead continues:

On the other hand, Putin would have grave difficulties surviving the loss of all Ukraine. The example of a popular revolution against a Moscow-leaning government is horrifying and destabilizing enough.... The consequence of a united Ukraine joining the West would be infinitely worse. Putin's dream of a Eurasian Union would suffer an irrevocable and decisive defeat. The loss of Crimea would infuriate Russian nationalists beyond endurance, and Putin would look helpless and weak in a political culture that worships only strength and success. [American Interest]

If Putin acts, it will be quickly and with some daring, on the assumption that the U.S. and Europe won't back up their rhetoric with force, Mead adds. And he has a point: Remember, "Ukraine matters much more in Moscow than it does in either Brussels or Washington."

Moscow can't be happy with Yanukovych's fall, but "the mess is very much Ukraine's own, and Russia has far less influence on it than is commonly appreciated," says Dmitri Trenin, director of the Carnegie Moscow Center, in The New York Times. "The most popular myth about Moscow's role in the Ukrainian crisis is that Yanukovych has been but a puppet" of Putin, but Putin is almost as frustrated as Ukrainians with the unreliable Yanukovych, "forever vacillating between the European Union and Russia."

Russia won't invade, but "despite what some Ukrainians suspect, Moscow is unlikely to try bringing about the breakup of Ukraine in order to annex its southern and eastern parts," either, Trenin says. "That would mean civil war next door, and Russia abhors the idea." Putin's most likely move is to "stand back and wait, while quietly favoring decentralization in Ukraine," with the eastern and western parts of the country in a loose alliance. That may be the best thing for Ukraine, he adds, but "Ukraine's 'February Revolution' may also be a blessing in disguise for Moscow, as it could help debunk the notion that Russia cannot be a great power without Ukraine as its junior partner."

Short of sending in troops, there are economic measures Russia can, and likely will, take "to give force to its rather vocal displeasure," says Julia Ioffe at The New Republic. "Before Yanukovich, when Ukraine was ruled by a pro-Western coalition, Russia often cut off the flow of gas to the country in order to exert political pressure." It may do that again. The interim government is making noises about joining the EU.

But Ukraine is bankrupt, corrupt, and politically divided, Ioffe notes. "Will the EU let the economically troubled country in to reward it for its choice? Or will it continue to string Ukraine along, much as it's done for the last decade?"

Even if Putin lets Ukraine go, it's not clear where the country will end up.