Chain Home — the Invisible Wall

Since the mid-1930s, the United Kingdom dreads above all the bomber: it is then believed that 'the bomber will always get through', and that no defence can stop a massive aerial attack. The physicist Robert Watson-Watt had nonetheless demonstrated, in 1935, that an aircraft could be detected at a distance thanks to radio waves.

At the head of Fighter Command, Air Chief Marshal Hugh Dowding follows these works closely. By the summer of 1939, a chain of detection stations — Chain Home — has been installed along the approaches to England, but the system remains young, fragile and never tested under fire. Linked to filtering and control centres, this nascent technology promises much, with no guarantee that it will hold on the day.

The doctrine is not settled. Should the whole of air defence be founded on this still-uncertain detection-and-control arrangement, concentrating the means on it, at the risk that it fail? Rely, as in the past, on permanent patrols and the network of ground observers? Or scatter the effort among several approaches without favouring any? The choice will determine the effectiveness of British fighter aviation if war breaks out.