At the beginning of August, with German invasion forces and troop barges being assembled on the French coast, the raids against the South coast of England were increased in size and number.
Believing that the British early warning system had been destroyed and the coastal towns sufficiently 'softened up' for an invasion, the Luftwaffe began the next stage of their plan.
On 13 August (called Adlertag or Eagle Day by the German High Command), massive raids began on the airfields of 11 Group. The aim was to destroy the RAF, either in the air or on the ground, in South East England. To put pressure on the British defences, the Germans sent high and low level raids to different targets at the same time.
Sometimes low level raids sneaked past the battered but still working radar stations, and the first warning the British fighter pilots had was bombs landing on their airfield. Particularly good in the low-level role was the Dornier Do 17 and its derivatives, several of their raids succeeded in achieving complete surprise and escaping any form of interception.
This pattern continued into September and the situation in 11 Group became desperate. Small civilian airfields were used in the emergency, as many RAF stations became badly damaged. The Spitfire and Hurricane could easily take off from grass fields, but the maintenance and spares supply situation became dangerously stretched. Ground crews working in the open suffered heavy casualties from the raids, and many manitenance facilities were destroyed in the bombing. Despite this, the crews kept the fighters as combat ready as possible, winning the Battle on the ground as the pilots were in the air.
Suggestions were made that the fighters should be pulled back north of the Thames, but Dowding and Park knew that this was exactly what the Germans wanted, effectively giving them air superiority over the intended invasion area. So the 11 Group squadrons stayed and fought for their lives.
To keep up the pressure, the Germans began night raids, to stop the defenders repairing damage overnight. On one night raid, some aircraft bombed civilian areas of London by mistake; a mistake which was to become a crucial turning point in the Battle. Attacks on civilian centers were something which had been specifically banned by Hitler, who was still hoping at this time that the hoplessness of the situation would cause the British to sue for a negotiated peace. The German High Command knew that widespread civilian casualties would only harden the resolve of the nation to fight on. In reply to this accidental attack, the British bombed Berlin. Fears grew that cities would be raided more often, so children were evacuated again in a second mass exodus to places of safety in the country, as they had been during the Phoney War of 1939.
However, just when it seemed that the country and 11 Group in particular couldn't continue for another day, the Germans changed their tactics.