After assaulting 11 Group's vitally-important sector stations for a fortnight, with considerable success, the Germans suddenly changed their tactics and launched an all-out attack on London. From their perspective, they felt that these targets had suffered sufficient damage and that, with time running out in which to launch a successful invasion, the most rapid conclusion to the Battle could be reached by an onslaught on the capital. It was certain to be defended in the greatest possible strength by Fighter Command and therefore offered, as no other objective could, the chance of a huge and decisive air battle. This decision was, however, a tactical error of such importance that it was arguably the turning point of the Battle of Britain.
In the air the day began quietly, though in the morning the Air Ministry issued its Invasion Alert No. 1 - "Attack Imminent". Just before 4pm the calm was broken as over 300 bombers and 600 fighters began heading for Kent and Sussex in two waves an hour apart. As this was the largest raid it had yet faced, Fighter Command alerted 10, 11 and 12 Groups. By 4.30pm, all 21 squadrons within 70 mile radius of London were in the air or ready to take off.
As the defences were expecting more attacks on the sector stations they were deployed accordingly and were too weak and widely dispersed to prevent raiders getting through to London. Some aircraft were able to intercept, but the sheer numbers of the escort fighters and their close proximity to the bombers prevented many from pressing their attacks home.
Defending squadrons finally got into position to take heavy toll of German aircraft as they returned home, but not before they laid waste to large areas of the London docks, Woolwich Arsenal, Beckton gasworks, West Ham power station and the oil storage tanks at Thameshaven. By then a second wave was approaching and it was now obvious that a major threat to London was in prospect. Although at least four squadrons engaged the raiders before they reached their targets, the strength of the German fighter force was such again as to beat off most challenges. This time Millwall and Commercial docks, Tilbury and Thameshaven were hit, together with the thickly-populated streets of the East End. The fires left burning were perfect markers for the bombers which continued to come right through the night and, indeed, for the next nine months.