Perhaps best known for the book he wrote with Stephen Walt, "The Israel Lobby and U.S. Foreign Policy," Mearsheimer is a proponent of great-power politics—a school of realist international relations that assumes that, in a self-interested attempt to preserve national security, states will preëmptively act in anticipation of adversaries. For years, Mearsheimer has argued that the U.S., in pushing to expand NATO eastward and establishing friendly relations with Ukraine, has increased the likelihood of war between nuclear-armed powers and laid the groundwork for Vladimir Putin's aggressive position toward Ukraine. Indeed, in 2014, after Russia annexed Crimea, Mearsheimer wrote that "the United States and its European allies share most of the responsibility for this crisis."