Stephen Cohen is one of America's top experts on Russia.  Cohen is professor emeritus of Russian studies and politics at New York University and Princeton University, and the author of a number of books on Russia and the Soviet Union.

Cohen says that the West is mainly to blame for the crisis in Ukraine:

This is a horrific, tragic, completely unnecessary war in eastern Ukraine. In my own judgment, we have contributed mightily to this tragedy. I would say that historians one day will look back and say that America has blood on its hands. Three thousand people have died, most of them civilians who couldn't move quickly. That's women with small children, older women. A million refugees.

Cohen joins other American experts on Russia – such as former U.S. ambassador to the Soviet Union, Jack Matlock – in this assessment.

Cohen also says that if Ukraine joins NATO, it will lead to nuclear war: