D-Day was important for several reasons. The most important reason was that it marked the first time in the war that Allied troops threatened Germany's control of Europe. Up until then, the fighting had been taking place very far away from the German border.
D-Day also marks the successful opening of a second front against the German dictator, Adolph Hitler, who at the time was attacking Russia. The Invasion also convinced Soviet Premier Joseph Stalin to stay in the war, now that his wish for a second front had been granted. Finally, the invasion was a way for America and Britain to keep Communist influence from spreading into Europe. Left unchecked, America and Britain feared that Russia might defeat Germany and gain a large foothold in Europe for communism to spread.