1.4.1 Principle of Distinction
The cardinal rule: the parties must distinguish between combatants and civilians (and between military objectives and civilian objects) — the attacks directed only against the former; the civilians' immunity (the AP I's core articles; the customary rule No. 1 of the ICRC study). The distinction's violations: the indiscriminate attacks; the direct targeting of civilians — the war crimes' core.
1.4.2 Principle of Military Necessity
The belligerent's licence-and-limit: only the measures necessary to achieve the legitimate military purpose — and not otherwise prohibited. The necessity's function: the permission's frame (the destruction of the enemy's forces, not the terror of the population); the tension with humanity (the classic balance: military necessity vs the requirements of humanity).
1.4.3 Principle of Proportionality
The collateral calculus: the incidental civilian harm must not be excessive relative to the concrete- and-direct military advantage anticipated. The proportionality's operation: the commander's ex ante assessment (the information reasonably available — the Aftermath's judgment's faces); the proportionality's debates (the weighting's incommensurability — the civilian lives vs the military advantage).
1.4.4 Principle of Humanity
The suffering's floor: the prohibition of suffering, injury or destruction not necessary for the legitimate military purpose (the St. Petersburg Declaration's 1868 line: "the only legitimate object... is to weaken the military forces of the enemy"). The humanity principle as the system's telos (the Martens Clause's twin).