The system-level approach and state-level approach are two distinct perspectives used to understand the causes of war in international relations theory.

System-Level Approach:

Focus: The system-level approach examines the broader international system and its structural characteristics to explain the occurrence of war.
Key Concepts: Key concepts in this approach include power distribution, polarity, alliances, balance of power, and security dilemmas.
Causes of War: According to this perspective, wars are often driven by systemic factors such as power imbalances, competition for resources, the absence of effective international institutions, and the anarchic nature of the international system. For example, scholars like Kenneth Waltz argue that the structure of the international system, particularly the distribution of power among states, influences their behavior and propensity for conflict. Power transitions between rising and declining states, as described by the power transition theory, can also increase the likelihood of war as the status quo power seeks to maintain its position while the challenger seeks to alter it.
Examples: The outbreak of World War I, often analyzed through the lens of systemic factors such as alliance systems, power rivalries, and arms races, is a classic example of how structural dynamics can contribute to conflict escalation.

State-Level Approach:

Focus: The state-level approach examines the internal characteristics, behaviors, and decision-making processes of individual states to understand the causes of war.
Key Concepts: Key concepts in this approach include state behavior, leadership, regime type, nationalism, domestic politics, and decision-making processes.
Causes of War: According to this perspective, wars are often driven by factors intrinsic to individual states, such as aggressive leadership, expansionist ambitions, nationalist sentiments, domestic political instability, or regime type. States may engage in conflict to pursue strategic interests, secure territorial gains, or divert attention from domestic problems. For example, the democratic peace theory suggests that democracies are less likely to go to war with one another due to shared norms, institutions, and mechanisms for conflict resolution.
Examples: The invasion of Iraq by the United States in 2003 is often analyzed through the state-level approach, focusing on factors such as the Bush administration’s foreign policy objectives, perceptions of threats, and domestic political considerations.

In summary, while the system-level approach emphasizes structural dynamics and systemic factors in explaining the causes of war, the state-level approach focuses on the internal attributes, behaviors, and decision-making processes of individual states. These two perspectives complement each other and provide insights into the complex interplay of factors that contribute to the outbreak and escalation of conflicts in the international system.