From Angelo Codevilla and Paul Seabury’s 1989 classic, War: Ends and Means:

Discrimination means that armed forces should fight armed forces and not ravage the enemy’s countryside, cities, or economy. While it is permitted to “starve out” an army, blockades of whole countries, such as the ones that kept food from Germany in World Wars I and II, have traditionally been considered unjust means of warfare because they do not discriminate between combatants and noncombatants. By the same token, it has been a traditional rule of Western warfare that when an army takes possession of a city with the intention of using it as a fortress by which to fight another army, both armies must allow the civilian population of the city to depart in an orderly manner. This is what General Charles George (“Chinese”) Gordon and his enemy, the Mahdi, Mohammed Ahmed, did before fighting the final battle of Khartoum in 1890. In other words, the corollary of the rule that armies may not make war on cities full of civilians is the rule that armies may not hide behind civilians.