“They push buttons,” says Timothy Snyder, a historian and the author of “Bloodlands,” who takes the long view of Russian aggression. “What button of ours are they pushing here? What are they trying to get us to do?” In a wide-ranging conversation, Snyder talks with David Remnick about the idea advanced by Vladimir Putin that Ukraine historically and rightfully belongs to Russia and about the dictator’s far-reaching goal of destabilizing nato.