Offensive and defensive realism are two prominent variants within the broader theory of realism in international relations, both sharing core assumptions but diverging significantly on the extent of power states seek and the primary drivers of their behavior in an anarchic international system.

At their core, both offensive and defensive realism agree that the international system is anarchic (lacking a central authority), states are the primary actors, and states are rational actors primarily concerned with their own survival. They also acknowledge that states possess some offensive military capability and that intentions of other states are uncertain, leading to a 'security dilemma' where actions taken by one state to increase its security can be perceived as threatening by others, leading to a spiral of arms races and mistrust.

Offensive Realism, most notably articulated by John Mearsheimer, posits that states are inherently power maximizers. In an anarchic system where survival is paramount and no state can ever be certain of another's intentions, the best way to ensure security is to achieve hegemony – to become the most powerful state in the system. Mearsheimer argues that states are constantly looking for opportunities to gain power at the expense of rivals and will engage in aggressive behavior when the benefits outweigh the costs. For offensive realists, states are never truly secure until they are the hegemon, and even then, they must work to prevent the rise of peer competitors. Therefore, states are driven by a desire for relative power and will always seek to maximize their share of world power, making conflict an ever-present possibility. The international system encourages, rather than discourages, aggressive behavior.

Defensive Realism, championed by scholars like Kenneth Waltz and Stephen Walt, offers a more restrained view of state behavior. While acknowledging the anarchic nature of the international system and the security dilemma, defensive realists argue that states primarily seek security, not power for its own sake. They contend that excessive power accumulation can be counterproductive because it tends to provoke balancing behavior from other states, thereby reducing rather than enhancing a state's security. Defensive realists believe that states are generally content with an 'appropriate' amount of power to ensure their survival and that the international system often provides incentives for cooperation and restraint. They emphasize that conquest is often costly and difficult, and that the costs of war often outweigh the benefits. Therefore, states are more likely to pursue defensive strategies, such as maintaining a sufficient military, forming alliances, and signaling benign intentions, rather than engaging in aggressive expansion. They suggest that the security dilemma can be mitigated, and that states can achieve security without constantly striving for hegemony.

In essence, the key distinction lies in their answer to the question: 'How much power is enough?' Offensive realists believe states always want more power, ideally hegemony, because security is never guaranteed. Defensive realists believe states seek only enough power to be secure, as excessive power can be self-defeating by triggering counter-balancing. This difference leads to varying predictions about the likelihood of conflict and the nature of state behavior in international politics.