Ukrainian President Petro Poroshenko, elected in May, has done a surprisingly good job of turning Ukraine's military into a more effective fighting force. Ukraine has received $20 million in American military aid, as well as intelligence and advice. But it is the new government's vetting of troops, decisiveness, and willingness to take casualties that have given its military an edge.

At the same time, pro-Russian separatists in eastern Ukraine have done a poor job of generating popular support. Instead of sparking a pro-Russian popular uprising, they have alienated much of the population in eastern Ukraine. "Separatists have worn out their welcome,” Pifer said. "The population is tired of this."

On Monday, Poroshenko accused Russian military officers of fighting alongside separatists. A missile that downed a Ukrainian transport plane carrying eight people near the border was probably fired from Russia, Ukrainian officials said. And on Friday, Ukrainian officials said a Grad missile attack that killed 23 Ukrainian soldiers showed that Russian arms are flowing to rebels as well. Russian officials deny supplying or aiding the rebels.

The situation remains volatile. Poroshenko and Putin could be drawn into a dangerous tit-for-tat cycle as the fighting intensifies. Tensions rose on Sunday when Russian officials threatened Ukraine with "irreversible consequences" after a Russian citizen was killed by a shell fired across the border from Ukraine. Kiev said the accusation that its forces had fired across the border was nonsense and suggested the attack could have been the work of rebels trying to provoke Moscow to intervene on their behalf. The rebels denied they were responsible.

Poroshenko’s strong performance since taking office has been a key factor in blunting Moscow, said Eugene Rumer, who served as the national intelligence officer for Russia and Eurasia at the U.S. National Intelligence Council from 2010 to January 2014. While offering to hold peace talks, Poroshenko has mounted decisive military action against the separatists, Rumer added. At the same time, Poroshenko has taken a softer line against Putin himself. "I was pleasantly surprised when he spoke out against further sanctions against Russia," said Rumer, now a Russia expert at the Carnegie Endowment for International Peace. "He was statesmanlike. He was signaling to the Russians that he’s willing to make a deal. And that is significant."

Matthew Rojansky, a Russia expert at the Woodrow Wilson Center, cautioned that time may be on Putin's side. He said Washington had a long history of focusing intently on any anti-Moscow uprising, typically known as a "color revolution," and then quickly shifting its attention elsewhere. "The bigger challenge is not going back to our bad old habit, which is to get very, very excited after color revolutions happen," Rojansky said, "and then completely forget that these countries exist." Putin knows that, he added, and is playing a long game to Washington’s short game.