The Royal Navy had not ignored the fast-craft threat until war began. British planning and construction had revived during the 1930s, building on the much larger motor-boat force created during the First World War. The strategic shock was the fall of Norway and France, which gave Germany new operating positions and enlarged the waters Britain had to defend. In this interpretation, Norway's delayed postal alert squandered the chance to mobilize against a German landing that depended heavily on surprise. France's defeat is likewise explained through command failure and the German advance through the Ardennes rather than a simple collapse of the fighting army. The existing coastal doctrine was usable, but the required number of patrol and escort craft rose far beyond the prewar assumption.
Chapter · 34:47-37:54
Why the fall of France changed the problem
Britain had begun small-craft preparations before the war, but the fall of Norway and France vastly increased the coast and convoy network that required defense.