I’m hoping ForeignAffairs.com won’t mind me copying part of the article taken from this page. to get John Mearsheimer’s excellent points across.
According to the prevailing wisdom in the West, the Ukraine crisis can be blamed almost entirely on Russian aggression. Russian President Vladimir Putin, the argument goes, annexed Crimea out of a long-standing desire to resuscitate the Soviet empire, and he may eventually go after the rest of Ukraine, as well as other countries in eastern Europe. In this view, the ouster of Ukrainian President Viktor Yanukovych in February 2014 merely provided a pretext for Putin’s decision to order Russian forces to seize part of Ukraine.