September 15, 1940. On this clear Sunday over southern England, the Luftwaffe throws everything it has at London, expecting to find the RAF beaten and bled white and easy to finish off. Instead they run into wave after wave of British fighters that were supposed to be gone, and by nightfall the whole German gamble to conquer Britain has quietly collapsed. This is the day the British call Battle of Britain Day, the climax of the whole battle. For weeks the German air force had been told the RAF was on its last legs, down to its final few dozen fighters, ready to be swept from the sky. So on the 15th they sent two huge daylight raids against London, hundreds of bombers with fighter escort, meant to force the last of Fighter Command up into a battle of annihilation and destroy it once and for all, clearing the way for the invasion. But the RAF was not finished, not remotely. Air Vice-Marshal Park committed his squadrons in strength, and as the German formations pushed toward London they were met not by a handful of survivors but by squadron after squadron of Spitfires and Hurricanes, hundreds of British fighters, coming at them the whole way in and the whole way out. Down in the Uxbridge bunker Churchill watched the plotting table fill with raids and famously asked Park what reserves he had left, and Park answered that there were none. Every single fighter was already in the air. And they held. The German bombers were scattered and mauled, their formations broken up, their bombs thrown down almost at random. Some fifteen hundred aircraft tangled over southern England through the day, and the Luftwaffe took its heaviest losses in weeks. But the real defeat was not in the numbers, it was in what the day proved. The RAF was supposed to be dead, and here it was, everywhere, stronger than ever. The Germans realized they could not win air superiority over England, and without air superiority there could be no invasion. Two days later Hitler postponed Operation Sea Lion, the invasion of Britain, indefinitely. It never happened. The 15th of September was the day the Battle of Britain was effectively won, the day the German plan to knock Britain out of the war fell apart in the sky over London. A handful of young pilots, "the Few," had turned back the tide. Britain would stay in the war, and because Britain stayed in, there would one day be a base from which to take Europe back. Everything that came later depended on the fact that this small island did not fall, and this is the day it was saved.