Realist theory claims that nations are motivated by considerations of power. In this impressive study, Haas contends that ideology has played a far greater role in shaping politics among major states over the last two centuries than realism expects. Searching for a systematic impact, Haas argues that the degree of ideological "distance" between great powers has been crucial in how their leaders define interests and perceive threats. It is not the content of a state's ideology -- defined as its prevailing political principles -- that matters so much as it is the ideological affinities or differences between states. Haas expects that, like birds of a feather, similar sorts of states, regardless of whether they are liberal, monarchist, communist, or fascist, will tend to flock together. Most of his book presents historical explorations of European great-power relations, as well as Sino-Soviet relations after 1949. Austria, Prussia, and Russia found common cause after the Napoleonic Wars, and the revolutionary ideologies of France and the Soviet Union made other states fear contagion and "falling dominoes." Of course, ideological foes did fight together against Hitler, and communist states have famously quarreled. In the end, Haas is most convincing in showing that power and ideology operate together in shaping perceptions of threats and interests.