The aerial bombing of cities in warfare is an optional element of strategic bombing which became widespread during World War I. The bombing of cities grew to a vast scale in World War II, and is still practiced today. The development of aerial bombardment marked an increased capacity of armed forces to deliver ordnance from the air against combatants, military bases, and factories, with a greatly reduced risk to its ground forces. Civilian and non-combatant casualties in bombed cities have variously been a purposeful result of the bombings, or unavoidable collateral damage depending on intent and technology. A number of multilateral efforts have been made to restrict the use of aerial bombardment so as to protect non-combatants.