Book excerpt: The principle of proportionality is one of the cornerstones of International Humanitarian Law. Almost all states involved in armed conflicts recognize that it is prohibited to launch an attack that is expected to cause incidental harm to civilians that exceeds the direct military advantage anticipated from the attack. This prohibition is included in military manuals, taught in professional courses, & accepted as almost axiomatic. Yet, the exact meaning of this principle is vague. Almost every issue is in dispute. Controversy is especially rife regarding asymmetrical conflicts, in which many modern democracies are involved. How exactly should proportionality be implemented when the enemy is not an army, but a non-state actor embedded within a civilian population? What does it mean to use precautions in attack, when almost every attack is directed at objects that are used for both military & civilian purposes?