As complex as World War II is, you could fill volumes and volumes of books with this answer. There were a lot of things going on in the war on a number of different fronts. The war was fought in the air, on the ground, and over the seas in most corners of the world. The question also infers that you are looking for one single event. That also complicates the answer.

There are many reasons that Germany lost World War II. Conventional wisdom suggests that the Battle of the Bulge in December of 1944 was the moment that ensured the Allies would win the contest. In a last ditch effort to break the American and British lines in the Ardennes Forest, Hitler gambled on a counterattack that would stop the Allied advance. The plan worked early on as the Germans created a "bulge" in the enemy lines. The Allies were caught by surprise at the beginning of the battle but soon recovered to push the Germans back. That was the last time the Americans and British would be slowed in their march towards Germany. At the same time, the Soviets were advancing from the East and the end was near for Hitler and the Nazis.