Neo-realism, also known as structural realism, emerged in the late 1970s as a significant theoretical development within the realist tradition of International Relations. It was primarily articulated by Kenneth Waltz in his seminal 1979 work, 'Theory of International Politics.' Neo-realism sought to refine and systematize classical realism by shifting the focus from human nature or the internal characteristics of states to the anarchic structure of the international system as the primary determinant of state behavior.

Emergence of Neo-realism: Classical realism, dominant after World War II, attributed state behavior to inherent human nature (e.g., lust for power, egoism) or the flawed nature of states. However, critics argued that this approach lacked scientific rigor and struggled to explain variations in state behavior. Neo-realism arose as a response to these criticisms, aiming to provide a more parsimonious and scientific explanation for international politics by focusing on the systemic level of analysis.

Waltz argued that the international system is characterized by anarchy – the absence of a central authority above states. This structural condition, rather than the internal attributes of states, compels states to behave in certain ways, primarily to ensure their survival. He sought to build a theory that could explain general patterns of international politics, rather than specific foreign policy decisions.

Basic Tenets of Neo-realism:

1. Anarchy as the Ordering Principle: This is the foundational tenet. The international system lacks a legitimate, overarching authority to enforce rules or protect states. This condition of anarchy is not chaos but rather the absence of hierarchy, meaning states must rely on themselves for security.

2. States as Primary Actors: While acknowledging the existence of non-state actors, neo-realism considers states as the most important units in international politics. States are viewed as unitary, rational actors, primarily concerned with their own survival and security.

3. Self-Help System: Due to anarchy, states cannot rely on others for their security. Each state must therefore prioritize self-help, meaning they must build up their own capabilities (military, economic) to protect themselves from potential threats.

4. Security Dilemma: The self-help nature of the system often leads to a security dilemma. When one state increases its security (e.g., by building up its military), it inadvertently decreases the security of other states, prompting them to also build up their defenses, leading to a spiral of arms races and mistrust, even if no state initially intended aggression.

5. Relative Gains: States are not just concerned with absolute gains (how much they benefit) but also with relative gains (how much they benefit compared to other states). This makes cooperation difficult, as states fear that their partners might gain more, potentially shifting the balance of power against them.

6. Balance of Power: States constantly seek to balance power, either internally (by increasing their own capabilities) or externally (by forming alliances) to prevent any single state from achieving hegemony. The international system tends towards a balance of power, which is seen as a mechanism for stability.

7. Structure over Unit-Level Explanations: Unlike classical realism, neo-realism emphasizes that the structure of the international system (anarchy, distribution of capabilities) is the primary driver of state behavior, rather than the internal characteristics of states (e.g., regime type, ideology, leadership).

8. Defensive Realism (Waltz's Version): Waltz argued that states are primarily defensive actors, seeking to maintain their security rather than maximize power. They aim for an 'appropriate amount' of power to ensure survival, as excessive power accumulation can provoke counter-balancing by other states.

Neo-realism provided a powerful and influential framework for understanding international politics, particularly during the Cold War, by offering a parsimonious explanation for recurring patterns of conflict and cooperation based on the structural constraints of the international system.