Russia withdrew one unit from its border with Ukraine last week in an apparent gesture to ease tensions. Tens of thousands of troops remain, officially participating in an exercise, but it's the kind of exercise that could quickly become something else, according to the commander of NATO's sizable military force.

　　"The force that is at the Ukrainian border now to the east is very, very sizable and very, very ready," said U.S. Gen. Phillip Breedlove.

　　And it faces a much smaller Ukrainian force, including many reservists and new volunteers.

　　NATO forces are relatively far away and officials have made clear they do not want to go to war with Russia over Ukraine. It seems that eastern Ukraine, with its large ethnic-Russian population is there for the taking, if Russian leaders decide they want it. But is a large area and difficult to defend.

　　Russia expert Igor Sutyagin, of London's Royal United Services Institute, does not think an invasion is imminent.

　　“To secure that, in a tight manner, you need such an enormous amount of troops," he said. "So it’s possible to invade, it’s possible to seize some parts of Ukraine, but it’s probably impossible to keep them.”

　　At the International Institute for Strategic Studies, retired British Brigadier Ben Barry says Russia could also face other challenges in eastern Ukraine, where the percentage of ethnic Russians is much lower than in Crimea.

　　“There would be the prospect of some sort of long-term Ukrainian people’s war, guerrilla movement, terrorism," he said, "something that would militarily oppose the occupation by the Russian forces in the long term.”

　　In addition, the international community would likely increase the economic and diplomatic isolation imposed on Russia because of the Crimea takeover. International moves so far have been criticized as weak.

　　But Russia expert Sutyagin says they are already costing Russia tens of billions of dollars and serve as a significant deterrent to further adventurism.

　　“Economic measures being undertaken by NATO members are much more serious than any sanctions or military moves,” Sutyagin said.

　　Experts also say Russia could achieve many of its apparent goals for Ukraine without invading again, including the weakening of the Kyiv government’s influence in the east, making the region economically dependent on Russia, and sending a message to NATO not to even consider putting troops on its border.