Counterinsurgency is "the totality of actions aimed at defeating irregular forces". The Oxford English Dictionary defines counterinsurgency as any "military or political action taken against the activities of guerrillas or revolutionaries" and can be considered war by a state against a non-state adversary. Insurgency and counterinsurgency campaigns have been waged since ancient history. However, modern thinking on counterinsurgency was developed during decolonization.

An insurgency is a violent, armed rebellion by small, lightly armed bands who practice guerrilla warfare against a larger authority. The key descriptive feature of insurgency is its asymmetric nature: small irregular forces face a large, well-equipped, regular military force state adversary. Due to this asymmetry, insurgents avoid large-scale direct battles, opting instead to blend in with the civilian population where they gradually expand territorial control and military forces. Insurgency frequently hinges on control of and collaboration with local populations.