The bombing of Britain in WW I had begun on January 19, 1915 when zeppelins dropped bombs around Yarmouth, killling six people. Certain German bombing operations of WW I had been suprisingly effective, especially when the Gotha bombers supplanted the zeppelins, the 'best' raids had inflicted 121 casualties per ton of bombs dropped and it was this figure that was used as a basis for predictions. The 1924 ARP Committee produced figures estimating for London there would be 9,000 casualties in the first two days and then a continuing rate of 17,500 casualties a week. These rates were considered conservative.
It was believed that associated there would be "total chaos and panic" and hysterical neurosis as the people of London would try to flee the city. To control the population draconian measures were proposed - bringing London under almost military control; physically cordoning London with 120,000 troops to force people back to work. A different government department proposed setting up camps for refugees for a few days before sending them back to London.
Thankfully these schemes remained on paper only and while estimates of potential damage remained high, the Air Raids Commandant (Major General H. Pritchard of the Royal Engineers) favoured a more reasoned solution. He discerned that panic and flight were basically problems of morale, if the people could be organized, trained and provided with protection then they would not panic. As part of this scheme the country was divided into regions each having its own command and control structure, in potentia at least.
The 1924 estimates were, during the build up to war, regularly revised upwards. In 1938 the Air Ministry predicted 70,000 casualties a week - in the first month of war the British government was expecting a million casualties, 3 million refugees and the majority of the capital destroyed. Measures to control this devastation were largely limited to grisly discussions about body disposal and the distribution of over a million burial forms to local authorities. The 1939 Hailey Conference had decided that providing deep shelters would lead to workers staying underground rather than working (this policy was reversed in 1940 with 79 tube stations opened for overnight shelters and specialised deep shelter construction begun).
As the war proved the effectiveness of aerial bombardment was, beyond the destruction of property, very limited. The Luftwaffe did not manage better than three casualties for every ton of bombs in many British cities and the expected social consequences hardly occured - moral remained high, 'shell-shock' was not at all common, the rates of other mental ailments fell. Crime did rise substantially.
During the Second World War the organization was responsible for the distribution of gas masks, pre-fabricated air-raid shelters (also known as Anderson shelters), the upkeep of local public shelters, the maintenance of the blackout and the rescue of people after raids.
The ARP was disbanded in 1946, to be reconstituted as the Civil Defence Corps in 1948.