ABSTRACT
The most significant change since the construction of the system of coastal gun emplacements, mobilisation points and London Defence Positions of the 1890's was the advent of the aircraft. Coastal defence research had led to a system that was converted to locate low-flying aircraft. The most familiar surviving defence structure is the pillbox, an invention of the Russians in the Russo-Japanese War of 19045 taken up by the Germans in the First World War and copied by the British. The fall of France and the departure for England of the last of the BEF from Marseilles on 20 June changed the circumstances for the defence of Britain. The relevance of the Royal Navy was limited by the obligation to protect convoys of merchant ships importing vital supplies, both foodstuffs and munitions, to Britain. Against the Royal Navy three forces had to be evaluated in determining what could be done to protect Britain against invasion.
