It was a battle of giants, and the largest military invasion of WWII: Nazi Germany against Communist Russia. But the war between Germany and Russia would be the first major land defeat for Hitler, and that defeat is considered the beginning of the decline of Nazi Germany.
In June 1941 Adolf Hitler broke the non-aggression pact signed in 1939 by Germany and the Soviet Union when he invaded Russia with an army of more than 3 million men, 7,000 artillery pieces, 3,000 tanks, and 2,500 aircraft [source: History]. Joseph Stalin, taken by surprise, found his military overwhelmed by the German onslaught. During the first week of the invasion there were 150,000 casualties among Soviet troops, and by October that year, German troops had taken 3 million Soviet prisoners of war [source: Rees]. German troops reached Moscow by December 1941, but the war was taking longer than anticipated -- clothing, food and medical supplies were wearing thin. When Soviet troops struck back hard to keep Moscow from falling, the Nazis failed to take Moscow.