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In the summer of 1940, Britain asked itself not 'will Hitler invade?' but 'when?' SEALION, the German invasion plan, provoked the construction of pillboxes, coastal defences, heavy-gun emplacements and anti-aircraft batteries, as well as the formation of the Home Guard and covert groups. Later, new dangers replaced SEALION: radar detection systems were expanded during the Blitz years, as were intelligence-gathering systems and listening posts. From 1944, Britain was again faced with a deadly threat, Hitler's 'Vengeance weapons'. This title provides a concise assessment of Britain's defensive systems, and presents a vivid picture of war on the home front.