Book excerpt: The story behind D-Day begins in 1939 when Nazi Germany, led by Adolf Hitler, attacked Poland and ignited World War Two. The following year, the Germans occupied France and Western Europe and launched a vicious air war against Britain. In 1941, they invaded the Soviet Union. Seemingly unstoppable, the Nazis now held virtually all of Europe. They imposed a ruthless system of control and unleashed the horror of the Holocaust. However, by 1943, the tide had begun to turn in favor of the Allies, the forces opposed to Germany. In the east, despite huge losses, the Soviets began to force the Germans back.