Civil defense, spelt civil defence outside the United States and now often called civil protection, is an effort to prepare civilians for military attack. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation, and recovery. Programmes of this sort were initially discussed at least as early as the 1920s but it was only after the threat of nuclear weapons became realized that they became widespread. As the intensity of the Cold War waned emphasis shifted from military attack to emergencies and disasters in general. In the context of Civil Defense in the United States this eventually led to the replacement of the United States civil defense with the Federal Emergency Management Agency. In some countries, the all-encompassing nature of civil defense is denoted by using name "total defence" such as the Swedish word totalförsvar. The name suggests committing all resources, hence the term total, of the nation to the defence. It can be compared to total war.

Mitigation is the process of actively preventing the war or the release of nuclear weapons. It includes policy analysis, diplomacy, political measures, and more military responses such as a National Missile Defense and air defense artillery. In the case of counter-terrorism, mitigation would include intelligence gathering and direct action against terrorist groups. Mitigation may also be reflected in long-term planning such as the design of the interstate highway system and the placement of military bases further away from populated areas.

Response consists first of warning civilians so they can enter Fallout Shelters and protect assets.

Staffing a response is always full of problems in a civil defense emergency. After an attack, conventional full-time emergency services are dramatically overloaded, with conventional fire fighting response times often exceeding several days. Some capability is maintained by local and state agencies, and an emergency reserve is provided by specialized military units, especially civil affairs, Military Police, Judge Advocates and combat engineers.

However, the traditional response to massed attack on civilian population centers is to maintain a mass-trained force of volunteer emergency workers. Studies in World War II showed that lightly trained (40 hours or less) civilians in organized teams can perform up to 95% of emergency activities when trained, liaised and supported by local government. In this plan, the populace rescues itself from most situations, and provides information to a central office to prioritize professional emergency services.

In the 1990s, this concept was revived by the Los Angeles Fire Department to cope with civil emergencies such as earthquakes. The program was widely adopted, providing standard terms for organization. In the U.S., this is now official federal policy, and it is implemented by community emergency response teams, under the Department of Homeland Security, which certifies training programs by local governments, and registers "certified disaster service workers" who complete such training.

In most of the NATO states, such as the United States, the United Kingdom or Germany as well as the Soviet Bloc, and especially in the neutral countries, such as Switzerland and in Sweden during the 1950s and 60s, many civil defence practices took place to prepare for the aftermath of a nuclear war, which seemed quite likely at that time. However, there was never strong civil defense policy because it fundamentally violates the Mutually Assured Destruction doctrine by making provisions for survivors. Also, a fully fledged total defense would have been too expensive. Above all, compared to the power of destruction a defense would have been inefffective. In the M.A.D. doctrine, there are not supposed to be any survivors for a civil defense system to assist (thus the acronym). However M.A.D. was not official government policy, and successive governments in the west sought to implement civil defence measures in the face of popular apathy.

During the Cold War, civil defense was seen largely as defending against and recovering from an attack involving nuclear weapons. After the end of the Cold War, the focus moved from defense against nuclear war to defense against a terrorist attack possibly involving chemical or biological weapons. After the September 11, 2001 attacks, in the United States the concept of civil defense has been revisited under the umbrella term of homeland security. The old US Civil Defence logo was used in the FEMA logo until recently and is hinted at in the United States Civil Air Patrol logo. To the contrary, in Europe the triangle CD logo continuous to be widely used.