Offensive vs defensive realism & the security dilemma in depth
Structural realism's internal split: Mearsheimer's offensive vs Waltz/Jervis's defensive realism, and the security dilemma that drives spirals and arms races.
A shared foundation, a contested conclusion
Structural realism (neorealism), codified by Kenneth Waltz in Theory of International Politics (1979), holds that the anarchic ordering principle of the international system—the absence of a world government—compels states to prioritise survival. From this single premise two rival schools emerge that agree on the diagnosis but split on the prescription: how much power should a rational state seek?
Defensive realism
Defensive realism, the position Waltz himself adopted, argues that the system rewards states that seek an appropriate, not maximal, amount of power. Because balancing coalitions form against any state that grows too strong—Waltz's 'balance-of-power tendency'—excessive expansion is self-defeating. The optimal grand strategy is to preserve the existing balance. Robert Jervis ("Cooperation Under the Security Dilemma," World Politics, 1978) and Stephen Van Evera supplied the supporting logic: under most conditions, conquest is hard, defence is easier than offence, and security is plentiful. Aggression therefore reflects domestic pathology or misperception, not structural necessity. The historical exhibit is Wilhelmine and Nazi Germany, whose bids for hegemony provoked the very encircling coalitions that destroyed them in 1918 and 1945.
Offensive realism
John Mearsheimer, in The Tragedy of Great Power Politics (2001), rejects the idea that the system supplies enough security. Because a state can never be certain of another's present or future intentions, and because military power is the ultimate arbiter, the only rational guarantee of survival is to maximise relative power—ideally to become a regional hegemon, as the United States did in the Western Hemisphere by roughly 1900. Great powers are revisionist by default, restrained only when the costs of expansion exceed the gains. Mearsheimer's five bedrock assumptions—anarchy, offensive capability, uncertainty of intentions, survival as the primary goal, and rational calculation—generate relentless competition. His doctrine of offshore balancing and his prediction that a rising China cannot rise peacefully follow directly from this premise.
Where the schools diverge
The practical fork is over the offence–defence balance. Defensive realists say technology and geography usually favour the defender, so restraint pays; offensive realists say states cannot reliably read that balance and must assume the worst. Both reject the liberal claim that institutions or commerce can override anarchy, distinguishing them sharply from neoliberal institutionalism (Keohane). For exam purposes, fix the names to the texts: Waltz/Jervis/Van Evera = defensive; Mearsheimer = offensive; both are structural and therefore distinct from classical realism's Morgenthau, who located the drive for power in flawed human nature (animus dominandi) rather than in system structure.