Following the evacuation of Dunkirk and the surrender of France, on 18th June 1940 Churchill made his “this was their finest hour” speech. He said that the Battle of France was over and that the Battle of Britain was about to begin. Hitler’s planned invasion of England (“Operation Sea Lion”) was planned for mid-September. He was advised that a sea-borne invasion could only be considered after Britain’s air defences had been neutralised.
The first phase of the Battle of Britain is said to have started on July 10th 1940 with what are known as the Channel Battles where the Luftwaffe tested the capabilities of the RAF and (almost as a sideline) destroyed a number of convoys in the Channel. In mid to late August attention turned to coastal airfields and in late August and early September the inland airfields were targeted before attention was turned to towns and cities. By late September Hitler realised that the battle was lost.