What kind of policy can the United States pursue that ensures its security while minimizing the likelihood of war? We describe and defend a realist theory of foreign policy to guide American decision makers. Briefly, the theory says that if they want to ensure their security, great powers such as the United States should balance against other great powers. They should also take a relaxed view toward developments involving minor powers and, at most, should balance against hostile minor powers that inhabit strategically important regions of the world. We then show that had the great powers followed our theory's prescriptions, some of the most important wars of the past century might have been averted. Specifically, the world wars might not have occurred, and the United States might not have gone to war in either Vietnam or Iraq. In other words, realism as we conceive it offers the prospect of security without war. At the same time, we also argue that if the United States adopts an alternative liberal foreign policy, this is likely to result in more, rather than fewer, wars. We conclude by offering some theoretically-based proposals about how US decision makers should deal with China and Iran.