Nazi Germany wanted to invade and occupy Britain to complete its conquest of Europe, so Hitler could complete his plan to move East. Operation Sea Lion was the codename of the invasion plan, and the barges and landing craft were assembled in readiness. But first the Roya Air Force had to be destroyed to stop it attacking the invasion fleet or providing air cover for British naval vessels that might intercept it at sea.
The planned attack on the RAF was codenamed Adlertag, or Eagle Day. It was postponed several times because of poor weather over the Channel and Southern England, but it finally took place on August 13th, 1940. Hundreds of German Luftwaffe planes attacked radar stations and fighter airfields in Southern England. They were met by fierce RAF resistance and faced a sophisticated air defence system that used radar, supplemented by the Royal Observer Corps, to pinpoint the numbers and direction of incoming enemy aircraft and send fighters up to intercept, Hurricanes to attack the bombers, and Spitfires to take on their Me109 escorts.
The German attack was marred by poor intelligence. They never really appreciated the significance of the radar stations, or understood how Britain's outnumbered fighters could be directed by ground control to attack specific flights without needing to mount costly defensive patrols. The WRAF personnel who manned the huge tables moved counters across the maps as information came in, enabling Dowding and Fighter Command to order planes to scramble to meet the attackers. The attackers had to use fuel flying to their targets, and only had minutes of effective flying left when they reached them before turning back. The RAF, on the other hand, were nearby, enabling their planes to stay longer in combat. Downed RAF pilots who survived could be rescued and sent rapidly back into service, whereas their German counterparts were taken prisoner.
Although damage was done on Adlertag, it did not make the major impact on the RAF's defensive capability that had been its aim. Poor intelligence sent some German planes to bomb the wrong targets, and crucially failed to pinpoint the factories where RAF fighters were made. German estimates had told Goering that Britain could produce roughly 250 fighters a month, whereas the actual figure was twice that.
Both sides overestimated enemy losses throughout the Battle of Britain, and on Adlertag the RAF claimed 78 Luftwaffe planes destroyed, whereas the actual number was 47 or 48 destroyed, and 39 severely damaged. The Germans claimed to have downed 70 British fighters in the air, with more fighters and bombers destroyed on the ground, whereas the actual British losses were less than one-third of those claims. The high Luftwaffe losses did not deter them from continuing their attacks throughout August and into September, until the Luftwaffe switched to night strategic bombing.
The failure of Adlertag and the ensuing Battle of Britain meant that Britain stayed in the war. It provided a base from which a future attack could be launched once the US had entered the war. It meant that Germany had two fronts to deal with when it attacked Soviet Russia. It is debatable if Russia could have defeated Germany had Britain been under Nazi occupation. Without the vital supplies taken by the Arctic convoys that helped Russia to sustain the war, and without Germany having to divert huge military resources to its second front in the West, it is conceivable, maybe even likely, that Hitler's armies would have subjugated the entire continent. And even if Russia had finally won a costly war of attrition, there would have been no allied troops in Western Europe to temper its colonial ambitions.
Thus the failure of Adlertag and the succeeding air attacks were instrumental in securing the preservation of freedom in Western Europe, and in ultimately defeating the evil scourge that sought to extinguish it. Among the brave airmen who fought in that battle was Antony Fisher, later Sir Antony, who afterwards went on to found the Institute of Economic Affairs, which itself played a major role (and still does) in preserving and extending freedom.