Civil defence consists of all the non-military actions that can be taken to reduce loss of life and property from enemy action. It embraces defence against all types of attack, including conventional explosive bombs or rockets, nuclear weapons and chemical or biological attacks. But in the missile era, civil defence presents many shortcomings. In the case of nuclear attack, short warning times preclude many civil defence measures (such as getting people into appropriate air-raid shelters) and many important factors are unpredictable. The behaviour of sheltered populations under extreme stress is largely unknown. The fire hazard from nuclear attack is only partly explored: a nuclear burst between an envelope of clouds above and snow below could increase the thermal effect (and resulting fires) by a large factor. There are inherent uncertainties as to when an attack may begin and when it may end, which frustrate civil defence actions. The detection of a fleet of approaching enemy bombers does not preclude the possibility of a simultaneous missile attack, nor that enemy submarines may be lurking offshore, both alternatives conveying threats with short warning times. Similarly, the explosion of a single nuclear weapon cannot be interpreted as the end of the attack for any specific area. More weapons may be in the offing at the very time that the desire to emerge from shelters to fight fires and attempt rescue work is at its peak.
Although in the early stages of World War I, Germany used rigid airships to attack England from the air and the Allies launched some counterattacks on Germany, it was not until World War II that the threat of aerial attack on cities became sufficiently great to call for organized civil defence planning. While a few special air-raid shelters were built in the UK and in Hawaii, civil defence tactics during the interwar years consisted principally of utilizing improvised shelters such as basements and subways. Germany also built special bunkers for a small fraction of its population, and these proved to be very effective in saving lives. Other civil defence tactics (in Great Britain and along the coasts of the USA) consisted of blackouts to reduce the night glow from city lights that could have served as guides to enemy pilots. The British government provided gas masks for its people, and practically all the countries involved in the war trained citizens in the elements of fire fighting, rescue and medical first aid.
The major, perhaps critical, difference between the civil defence situation in World War II and that which has confronted the world since 1950 is that while the relatively small weapons of World War II afforded some 'learning time' - people could learn by experience that shelters were safer than ordinary buildings and civil defence volunteers could be recruited and trained after the war had begun - no learning time is allowed by nuclear weapons that can destroy whole metropolitan areas at one blow. There is no opportunity to learn from repeated attacks because the first attack in all probability, will accomplish its mission.