Battle of Britain (1940): Hardest day · 360

June 1940 left Britain expecting invasion. The Channel was a moat only if the air above it was contested. Sea Lion was a sketch that needed a sky. Dowding had already fought to keep fighters in Britain during the French collapse -- a political-military preface. August attacks on 11 Group fields (Biggin, Kenley, Hornchurch) were the dangerous week. Park's tactics -- meet them forward, do not empty the fields -- remain a historians' argument with Leigh-Mallory's Big Wing.

Pilot fatigue and the training pipeline were the real clock, not a single "hardest day" postcard. The Luftwaffe's turn to the capital eased pressure on the sector stations and opened the night Blitz. Whether this was revenge, a hoped-for RAF final battle, or both, it changed the campaign's object.

15 September -- later Battle of Britain Day -- became the ritual date. The 1969 film, St Paul's in smoke, and the Capel-le-Ferne memorial taught a nation. Polish and Czech squadrons (303, 310) were late additions to popular English tellings that had been too English. A defensive campaign that made Overlord imaginable and gave English a "Few" vocabulary later wars borrowed.