Former U.S. Ambassador to Russia Michael McFaul told reporters Monday continued escalation by Russia into eastern Ukraine could lead to years of "guerrilla warfare."
Russian President Vladimir Putin faced little resistance going into the Crimean peninsula, McFaul said. He recounted how quickly events moved in the region; first there was the Russian occupation, then the referendum in which Crimeans voted to join Russia, and finally the quick annexation that became official last Friday.
However, if Putin decides to go further into eastern Ukraine, McFaul predicted he will face backlash from a population far less tied to Russia.
"If he does move in there, I think there will be resistance," McFaul told reporters on a conference call hosted by Foreign Affairs magazine. "I doubt it will be organized military resistance, but you'll have guerrilla warfare for months, if not years. So he's got to calculate that into his analysis, as well."
Over the past few days, Western leaders have expressed concern Russia's build-up of troops along the border of eastern Ukraine. On Sunday, Ukrainian Foreign Minister Andrii Deshchytsia told ABC News the chances of war with Russia were "growing," after NATO's top military commander said Russia had amassed a "very, very sizeable" and "very, very ready" force at the border.
During a meeting with Russian Foreign Minister Sergei Lavrov, U.S. Secretary of State John Kerry "expressed strong concern about the massing of a large number of Russian forces on the border and the treatment of Ukrainian military forces," the State Department said.
It isn't clear what specific steps the U.S. would take in the event Russia continues to escalate the situation in Ukraine. McFaul said he thought the sanctions President Barack Obama levied on Putin's inner circle last week were having their intended effect. However, McFaul also said he didn't think those sanctions would deter Putin if he wanted to move into eastern Ukraine.
McFaul also provided an ominous readout as to what the U.S. could do to fully deter Putin.
"When it comes to deterring Russian aggression in Europe," he said, "America's track record is pretty poor. ... Whenever Moscow has decided it's going to use military force in eastern Europe, our instruments for deterring that have not worked."