Civil defense, civil defence (see spelling differences) or civil protection is an effort to protect the citizens of a state (generally non-combatants) from military attack. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation and recovery. Programs of this sort were initially discussed at least as early as the 1920s and were implemented in some countries during the 1930s as the threat of war and aerial bombardment grew. It became widespread after the threat of nuclear weapons was realized.