The Principle of Proportionality
Balancing military advantage against expected civilian harm in the conduct of hostilities.
The Proportionality Rule
The principle of proportionality prohibits attacks that may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated. In other words, even a lawful military target cannot be attacked if the expected civilian harm is disproportionate to the military gain.
Proportionality does not require zero civilian casualties. IHL accepts that civilians may be harmed as an incidental consequence of attacks on military objectives (collateral damage). What it prohibits is excessive collateral damage: harm that is out of proportion to the military advantage sought. This assessment must be made by the commander before ordering the attack, based on the information reasonably available at the time.
