International relations scholars have tended to focus on realism's common features rather than exploring potential differences. Realists do share certain assumptions and are often treated as a group, but such a broad grouping obscures systematic divisions within realist theory. Recently, some analysts have argued that it is necessary to differentiate within realism. This article builds on this line of argument. The potential, and need, to divide realism on the basis of divergent assumptions has so far been overlooked. In this article I argue that realism can be split into two competing branches by revealing latent divisions regarding a series of assumptions about state behavior. The first branch is Kenneth Waltz's well-known neorealist theory; a second branch, termed here “postclassical realism,” has yet to be delineated as a major alternative but corresponds with a number of realist analyses that cohere with one another and are incompatible with Waltzian neorealism.

Unipolarity is arguably the most popular concept used to analyze the U.S. global position that emerged in 1991, but the concept is totally inadequate for assessing how that position has changed in the years since. A new framework that avoids unipolarity's conceptual pitfalls and provides a systematic approach to measuring how the distribution of capabilities is changing in twenty-first-century global politics demonstrates that the United States will long remain the only state with the capability to be a superpower. In addition, China is in a class by itself, one that the unipolarity concept cannot explain. To assess the speed with which China's rise might transform this into something other than a one-superpower system, analogies from past power transitions are misleading. Unlike past rising powers, China is at a much lower technological level than the leading state, and the gap separating Chinese and U.S. military capabilities is much larger than it was in the past. In addition, the very nature of power has changed: the greatly enhanced difficulty of converting economic capacity into military capacity makes the transition from a great power to a superpower much harder now than it was in the past. Still, China's rise is real and change is afoot.

The development of the concept of soft balancing is an attempt to stretch balance of power theory to encompass an international system in which traditional counterbalancing among the major powers is absent. There are two fundamental faws, however, in current treatments of soft balancing: the failure to consider alternative explanations for state actions that have the effect of constraining the United States, and the absence of empirical analysis of the phenomenon. A comparison of soft balancing and four alternative explanations in the main cases highlighted by proponents of the concept-Russia's strategic partnerships with China and India, Russian assistance to Iran's nuclear program, the European Union's efforts to enhance its defense capability, and opposition to the U.S.-led Iraq war in 2003—reveals no empirical support for the soft-balancing explanation. The lack of evidence for the relevance of balancing dynamics in contemporary great-power relations indicates that further investments in adapting balance of power theory to today's unipolar system will not yield analytical dividends.

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After sixty-five years of pursuing a grand strategy of global leadership—nearly a third of which transpired without a peer great power rival—has the time come for the United States to switch to a strategy of retrenchment? According to most security studies scholars who write on the future of U.S. grand strategy, the answer is an unambiguous yes: they argue that the United States should curtail or eliminate its overseas military presence, abolish or dramatically reduce its global security commitments, and minimize or eschew efforts to foster and lead the liberal institutional order. Thus far, the arguments for retrenchment have gone largely unanswered by international relations scholars. An evaluation of these arguments requires a systematic analysis that directly assesses the core claim of retrenchment advocates that the current “deep engagement” grand strategy is not in the national interests of the United States. This analysis shows that advocates of retrenchment radically overestimate the costs of deep engagement and underestimate its benefits. We conclude that the fundamental choice to retain a grand strategy of deep engagement after the Cold War is just what the preponderance of international relations scholarship would expect a rational, self-interested leading power in America's position to do.