Classical Realism, championed by Hans Morgenthau, and Neorealism (or Structural Realism), developed by Kenneth Waltz, represent two distinct yet related schools of thought within international relations theory. Their major differences lie in their core assumptions about the causes of state behavior and international conflict.

Morgenthau's Classical Realism posits that international politics is driven by an inherent 'will to power' rooted in pessimistic human nature. States, led by individuals with this nature, perpetually seek to maximize their power relative to others. Anarchy in the international system is acknowledged but seen as a condition that permits this human drive for power to manifest, rather than being its primary cause. The focus is on the unit level (states and their leaders), emphasizing diplomacy, national interest defined in terms of power, and the balance of power as key concepts. Power is viewed as multidimensional, encompassing military, economic, and psychological aspects.

Waltz's Neorealism, in contrast, downplays human nature as an explanatory factor. Instead, it argues that the structure of the international system—specifically its anarchic nature—is the primary determinant of state behavior. States are considered unitary, rational actors whose main goal is survival. In an anarchic system, states are compelled to self-help, leading to a security dilemma where one state's efforts to enhance its security can threaten others. Waltz focuses on the system level, explaining patterns of international outcomes (e.g., why wars occur, why alliances form) rather than specific foreign policy decisions. Power, primarily defined by military and economic capabilities, is seen as a means to an end (security), not an end in itself. The distribution of capabilities (polarity) is central to understanding systemic stability.