The Second Sino-Japanese War (1937–45) and its aftermath presents a challenge to systemic-level International Relations theories about the relationship between economic interdependence and war. Critiquing that literature and turning instead to the domestic level, the chapter shows how Chinese and Japanese conceptions of the relationship between economics and security – first developed in response to the crisis of nineteenth-century Western imperial coercion in East Asia, and later amplified in the context of Cold War East Asia – fused the Chinese and Japanese economies in the lead up to, during, and in the decades following the Second Sino-Japanese War. In so doing, the chapter demonstrates that economic interdependence between China and Japan has grown explosively during, after, and because of war, and that perceptions of insecurity have motivated closer economic ties between China and Japan.