Scholars in international relations have failed to note a paradox about the balance of power: the concept of checks and balances and equilibria underlie classical Liberal constitutional and economic theory. Interest balancing interest lies at the core of the Liberal solution to the problem of anarchy, power, and human nature, whether in politics, economics, or the international sphere. Liberal scholars have adopted instead a rationalist utilitarian or a normative democratic approach. At the same time, Realists in international relations predict a balance, which realist scholars in domestic politics, like Schattschneider, have effectively questioned. This intellectual confusion denies Liberal theory a robust view of international politics, not least because the balancing principle is erroneously rejected as conservative. The confusion also undermines the coherence of Realist theory, which has hitherto tried to accommodate opposing predictions (balance of power and power concentrations) under one paradigm. I offer an explanation of how this conflation of theories arose. Conflating Liberalism with idealism leaves Realism as the only prudent alternative in international politics. The relation between the two theories is not zero-sum: both capture important aspects of international dynamics, and each can ignore the other only at serious cost.Deborah Boucoyannis is Lecturer in Social Studies at Harvard University (firstname.lastname@example.org). For comments and suggestions over the time the article has been in gestation, the author thanks Bear Braumoeller, Tim Crawford, Daniel Drezner, Nisha Fazal, David Grewal, Arman Grigorian, Mark Haas, Russ Hamilton, Michael Heaney, Jacques Hymans, Seth Jones, Andreas Kalyvas, Jacob Levy, Jack Levy, John Mearsheimer, Jennifer Mitzen, Kevin Narizny, Jacob Schiff, Jack Snyder, Kenneth Waltz, Alex Wendt, Bill Wohlforth, and the participants at the Political Theory and PISP workshops at the University of Chicago. Special thanks are due to Monica Toft, John Ikenberry, and Jennifer Hochschild for their support of the article; it was mostly written with the generous support of the Olin Institute for Strategic Studies at Harvard University.