Following the Battle of Britain, Hitler abandoned his invasion plans and turned his armies against his former ally, the USSR. Operation Barbarossa began on 22 June 1941, and turned into the most costly and devastating military campaign in history. 4.5 million troops attacked along a 1,800 mile front.
Ultimately, it failed - just as Napoleon's invasion of Russia failed - because Hitler had underestimated both the strength of Soviet resistance and the ferocity of the Russian winter. The German army lost over 700,000 men in the first five months.
However, unlike Napoleon, the German army remained in possession of significant tracts of the USSR, and long after Operation Barbarossa concluded in January 1942 the two armies clashed over cities such as Voronezh and Stalingrad. Over 5 million Axis troops died on the Eastern Front, along with 9 million Soviet troops. Civilian deaths have been estimated at 15 million. It was a war of extermination.