The Battle of Britain was arguably one of the most important events during World War II. It arguably marked the point where the tide definitively turned in the Allies favor. Before then the Nazis had enjoyed unmatched success as they conquered and subdued most of continental Europe. After the Battle, they lost more territory than they gained even though the War dragged on for four more years.
The conquest of Europe by Nazi Germany under Adolf Hitler was chiefly motivated by a stated need to carve out the continent as a utopian Aryan homeland. Supported by a ruthlessly efficient war machine, the Germans managed to subdue most of Europe including France, Poland, Denmark, and Norway. It happened over the first year and a-half of the War, which testifies about Germans dominance. Britain therefore represented the last remaining bastion of resistance in the West.
Unlike most other previous conflicts, the Battle of Britain was one of the first to be almost entirely fought in the air. The German Luftwaffe, under the command of Hermann Goring was one of the largest air forces ever assembled. The German Luftwaffe, a massive assemblage of fighter planes and bombers squared off against the Royal Air Force and remnants of French forces.
Between July and October 1940, the Germans embarked on a campaign of heavy bombardment on English towns and cities. These attacks were based out of multiple locations in France and Germany. Their aim was clear; subdue the British from the air in preparation for a massive ground attack that would see the Germans stake the claim as rulers of all of Europe. The British countered by directly engaging the invaders in the skies above England and the English Channel.
By the end of October 1940, the British and their allies had claimed a definitive victory. Their victory was enabled by a combination of factors. The British were more resilient in defending their homeland than the French had been. The Germans underestimated their capacity for self-defense and assumed that they would capitulate as easily as the French had fallen. The German aircraft were inferior in air combat when matched against the British ones since they were more suited for a support role rather than as a primary attacking force. Without the immense firepower of the German ground armies in support, the Luftwaffes weaknesses were found out. Additionally, Britain benefitted immensely from American aid in the form of food and weapons that were shipped across the Atlantic. Finally, the Germans were stretched thin from having to fight the war on two fronts.
The British victory undoubtedly changed the course of the War. Who knows what would have happened if the Nazis had been successful? It represents one of the few instances in history when good unambiguously triumphed over evil.