On September 7, 1940, more than 300 German bombers escorted by several hundred fighters came up the Thames in the late afternoon and hit the docks, the Surrey Commercial Docks, Woolwich Arsenal and the West Ham power station. A second wave arrived after dark. Around 430 people were killed in the first day and night, and the fires in the timber yards and warehouses along the river were visible from the air for hours.
This was the opening of the Blitz, and it ran for 57 consecutive nights from September 7 to early November, then continued in a second phase against other cities and against London until May 1941. Coventry went on November 14. The total civilian death toll across the whole Blitz is usually put at roughly 40,000, and the destruction in the East End was concentrated because the docks were both a target and surrounded by housing.
The reason the first raid came on this day was a chain of accidents and retaliation rather than a plan. German aircraft had bombed central London on August 24 after straying off target, and the RAF had answered by bombing Berlin on August 25. Hitler and Goering then ordered the Luftwaffe to shift from attacking Fighter Command's airfields and sector stations to attacking London, on the calculation that bringing the RAF up in defence of the capital would destroy it.
It did not. Fighter Command's system of radar, ground control and sector airfields had been under severe strain during August, and the shift bought it the breathing space to repair airfields, rest pilots and replace aircraft. The Germans had no idea. The chain had come that close to breaking.
What the raids did change was everything downstream of the air war. Shelter policy, evacuation, and the decision to accept attacks on cities by both sides were all settled in these weeks. The government had been reluctant to open deep shelters for fear of undermining morale; once the East End had been hit, policy moved faster than the paperwork. The word Blitz was already in the newspapers by the end of September.