"HG Wells style science fiction" isn't a bad starting point for inter-wars fears of aerial bombardment; over the course of the First World War aeroplanes had progressed from barely carrying their crew at the start of the conflict to more than a ton of bombs at the end of it; Zeppelins and heavy Gotha bombers had bombed London with little material effect but considerable psychological impact. It didn't require too much extrapolation to predict the impact of ever larger fleets of faster, heavier bombers unleashing explosive, incendiary and poison gas bombs on cities, sparking fears of a "knock-out blow", a massive, rapid, devastating attack from the air that could not be defended. From JFC Fulller's The Reformation of War from 1923, for example: "I believe that, in future warfare, great cities, such as London, will be attacked from the air, and that a fleet of 500 aeroplanes each carrying 500 ten-pound bombs of, let us suppose, mustard gas, might cause 200,000 minor casualties and throw the whole city into panic within half an hour of their arrival. Picture, if you can, what the result will be: London for several days will be one vast raving Bedlam, the hospitals will be stormed, traffic will cease, the homeless will shriek for help, the city will be in pandemonium. What of the government at Westminster? It will be swept away by an avalanche of terror."

As Brett Holman notes in The Next War in the Air "... there was little difference in content and, to an extent, style between knock-out blow scenarios in fiction and non-fiction", so alongside military theorists there were Future War novels with bombing as a major theme including Neil Bell's 1931 The Gas War of 1940, S. Fowler Wright's 1936 Four Days' War, and Wells himself with The Shape of Things to Come in 1933.

Faced with such a prospect there were two schools of thought in inter-war Air Raid Precaution (ARP) policy. One called for large Deep Shelters, about the only way to guarantee protection from increasingly destructive bombs. The other favoured the principle of dispersal, avoiding large concentrations of people in favour of extensive domestic shelters with smaller public shelters where necessary. Arguments against deep shelters included the risk that they would become death traps if their power and ventilation systems were destroyed or compromised by gas, and the psychological effect on those taking shelter (the member of one committee recalled taking shelter during the First World War "... where a large number of people had gathered: they were in a state of high nervous tension and one or two people had hysterics. I found my own personal fear mounting to such an extent that I had to leave the shelter and go out into the street where the bombing was taking place, in order to get my courage back"), including the fear that people would not emerge while there was a risk of bombing. The policy of dispersal won out, the enormous costs of deep shelters being another issue.

In line with such policy, and to prevent interference with transport, the London Underground was officially closed to shelterers. In response, according to Richard Overy's The Bombing War, "several thousand Londoners bought tickets for the Underground and stayed put in the stations and tunnels" on September 7th 1940, the first night London was heavily bombed. The following night a large crowd gathered at Liverpool Street Station and refused to leave, the doors eventually being opened to them. At other stations "Minor confrontations occurred, orchestrated in some cases by Communist party activists, between crowds waiting to go below and Underground officials whose instructions were to lock the entrances once a raid began" (Geoffrey Field, "Nights Underground in Darkest London: The Blitz, 1940-1941", International Labor and Working-Class History, No. 62).

The government attempted to dissuade people, putting out a notice in newspapers on September 19th exhorting people "to refrain from using tube stations as air raid shelters except in case of urgent necessity", but made no serious attempts to lock down stations or evict people by force; on September 21st Home Secretary John Anderson informed Churchill that the use of the tube for shelter was a fait accompli and that only military assistance could prevent it, something that public opinion would not allow. By 25th September the use of tube stations as shelters was officially recognised. By this stage it was apparent that aerial bombing, though terrible, was not capable of inflicting the annihilating knock-out blow feared before the war.