International humanitarian law (IHL) prohibits an attack expected to cause incidental civilian death, injury or damage that would be excessive in relation to the concrete and direct military advantage anticipated. The rule assesses expected harm before the attack; a later casualty count alone neither proves nor disproves a violation. It applies alongside the separate duties to distinguish military objectives from civilian objects and take feasible precautions.
The legal test is specific to the attack
Article 51(5)(b) of Additional Protocol I states the prohibition, and the International Committee of the Red Cross identifies it as customary law in international and non-international armed conflicts. The comparison is between the expected incidental harm and the concrete, direct military advantage anticipated from the attack, as the ICRC summarizes in Customary IHL Rule 14. General political aims cannot replace an identified military advantage. The law does not prescribe a numerical formula or make civilian casualties a standalone ratio.
The assessment is made on information reasonably available to those planning or deciding the attack at the time. Subsequent evidence may help establish what was known or reasonably expected, but hindsight does not replace that inquiry. Nor does a mistaken estimate automatically make an attack lawful: the expected harm, anticipated advantage, target verification, and feasible precautions all matter.
Proportionality works with precautions
Article 57 requires those planning or deciding an attack to do everything feasible to verify the target is a military objective, choose means and methods to avoid or minimize incidental harm, and refrain from an attack expected to cause excessive harm. An attack must be cancelled or suspended if new information changes that assessment. When a choice among objectives offers similar advantage, the objective expected to pose the least danger to civilians must be selected.
These duties matter in urban fighting, where Hamas and other armed groups operate among civilian buildings. Their conduct cannot erase civilians’ protection or relax Israel’s obligations. For the distinct rules on civilian sites and warnings, see civilian protection in urban targeting. For Israel’s review systems, see how Israel reviews targeting decisions.
Casualty totals are evidence, not the legal test
Observed deaths and damage can be relevant evidence in a later investigation, but proportionality asks what incidental harm was expected and what military advantage was anticipated when the attack was planned or decided. A high toll does not by itself establish excessiveness, and a low toll does not cure an attack that was expected to be excessive or directed at civilians. The legal question remains the specific attack, its information picture, and the precautions taken.
The phrase “attack as a whole” must not be used to pool unrelated strikes or substitute a campaign’s broad goals for the advantage of a particular attack. The relevant advantage must be tied carefully to the attack being assessed.