In “Anarchy and Struggle for Power,” John Mearsheimer presents his theory of offensive realism with regard to international relations between nation-states, which he refers to as “great powers.” He argues that great powers constantly strive for the goal of hegemony, or dominance, over each other by becoming more and more powerful.

Mearsheimer bases his theory on five basic assumptions about how the “international system” works. These assumptions, he asserts, are sound and realistic, and taken together, they contribute to the “explanatory power” of his theory about why states often tend toward aggression. The first assumption is anarchy in the world system. Anarchy here does not refer to chaos or disorder but rather a lack of central authority. Second, Mearsheimer assumes that great powers possess the capacity to hurt each other through their “offensive military capability.” Third, states' intentions toward each other are never certain and change rapidly. Fourth, states want to survive, and this desire for security motivates all their actions. Fifth, states are “rational actors” and pay close attention to other states as they plan how to survive. These five assumptions, Mearsheimer explains, “create powerful incentives” for offensive behavior.

Mearsheimer next identifies three possible “patterns of behavior” that arise in great powers. The first is fear. States are continually scared and suspicious of each other, for they know that other powers are capable of attacking at any time, and there is no central authority to call on for help. The potential consequences of such attacks are serious, including war and conquest. Second, states can only count on themselves for their own survival, so they establish patterns of self-help behavior and do whatever they can to remain in existence. Third, states seek maximum power: each one wants to become the most powerful among its rivals. Each one longs to be the hegemon, the dominant state, in the system. Therefore, each tries its best to swing the balance of power in its favor, even if that sometimes entails aggression.

This “pursuit of power” will continue, Mearsheimer explains, until one state becomes a hegemon. States are not satisfied with merely having greater power; they must dominate. They can never know for sure how much power they need to be secure nor how the picture will change in the future. Even if a state has little chance of becoming a hegemon, it will still strive after as much power as possible, taking advantage of other states as necessary and trying to defend itself against aggression from other states. This leads to a “constant security competition” between states. Indeed, a “security dilemma” arises when one state's efforts to increase its security actually decrease other states' security.

Mearsheimer points out that states focus more on relative power than absolute power. States are always comparing themselves to other states, making sure that they have more power then their rivals. This is relative power; even small power gains are important if one state is gaining more than another. Absolute power, on the other hand, strives after power for its own sake rather than in comparison with the capabilities of others.

Even with this strong desire to attain power, states are not “mindless aggressors.” Rather, they carefully calculate, according to their particular capacities, whether to take offensive action. States with more power and a stronger military will be more likely to take an aggressive stance than weaker states, which would rather defend the current balance of power until they can take an advantage for themselves. States, therefore, carefully consider whether the benefits of offensive action outweigh the “costs and risks.” Such calculations are made more difficult by rivals' misrepresentations of intentions and strength, by uncertainty about military power (one's own and others'), and by doubt about the determination of rivals and allies.

Mearsheimer acknowledges that other political theorists disagree with his offensive realism theory. These defensive realists argue that aggression often fails and that states tend to be content with the current balance of power and actually are often benign toward each other. Mearsheimer counters that history simply does not support the defensive realists position, for states frequently take the offensive, and aggression actually does often pay.

The author then turns his attention to a discussion of hegemony. He defines a hegemon as “the only great power in the system”—a state that dominates the others. While no global hegemon exists in the current world system, the United States is the regional hegemon of the Western Hemisphere. Mearsheimer argues that in the present situation, no state will become a global hegemon, mostly because of world geography. Oceans serve as a barrier to conquest. States have, however, long endeavored to become regional hegemons, and the United States has stepped in several times to prevent such an occurrence. Ideally in the modern world, a state seeks to become the only regional hegemon, for it is thus the “status quo power” in the world.

With all this rivalry in the world, states focus on two factors: power and fear. The level of fear dictates the intensity of the security competition between states and the possibility for aggression. Fear arises because states have the capacity for military actions, are uncertain of other states' intentions, and cannot turn to a higher authority for help. Therefore, they try to maximize their power to both minimize their own fear and to maximize the fear of their rivals.

Mearsheimer differentiates potential power (determined by a state's population and wealth) from actual power (military forces). Armies are the key to actual power, for they conquer and control territory. The author further identifies three power factors that affect fear among states. First, he explains that powers with nuclear capabilities will actually be less afraid of each other because of the widespread reluctance to use nuclear power. Second, oceans decrease fear among states separated by them. Third, the “distribution of power” affects the level of fear among states. An “unbalanced multipolarity” (a multi-state system with a possible hegemon) generates the most fear, especially if there is a large power gap between the potential hegemon and the “second most powerful state.” Bipolarity (two powerful states in a system) leads to the least fear, while a “balanced multipolarity” (a multi-state system without a possible hegemon) falls in between the other two possibilities. A state's level of fear is also affected by the military capabilities of their rivals as well as their “latent power” of population and wealth.

States work toward one major goal: their own survival. They may also seek economic growth, the promotion of an ideology, national unification, and human rights advancement. These goals often combine with the goal of survival, but the goal of survival will always take precedence over the others.

Mearsheimer makes the point that while states pay “lip service” to world peace, they do not actually work toward a peaceful world order. Rather they strive to maximize their own power and only promote world order if it is in their own favor. Self-interest always prevails. Further, states rarely agree on methods for increasing peace, and they remain uncertain if their peace-making efforts will actually work and at what cost.

States do cooperate, but cooperation is “sometimes difficult to achieve and always difficult to sustain.” States want to make sure that they will profit by cooperation and that gains will be distributed to their advantage. They are also suspicious about cheating on the part of their rivals. States form alliances against common enemies when it works to their own advantage, but their essential competition remains dominant.

Mearsheimer concludes by reasserting his theory that great powers “think and act offensively” and “seek hegemony” because they function in an anarchic system and must aggressively pursue their own survival.