Operation Barbarossa

Operation Barbarossa (, named after Frederick I) was the code name for the European Axis invasion of the Soviet Union during World War II. It began on 22 June 1941. More than 3 million men attacked along the 2,900 km front, which made it the largest military invasion in human history. It also involved 600,000 motor vehicles and 750,000 horses.
In the Molotov-Ribbentrop pact of 1939 Germany and the Soviet Union agreed not to fight each other. Planning for the German surprise attack started in December 1940. The attack was to start on May 15, and its main objective was to destroy the Soviet military power. Some Nazi ideological goals were included, as were Soviet deposits of natural resources, which would be helpful to keep fighting the war against the Allies.
Even though the Soviet Red Army suffered huge losses, the attack did not complete its objectives. The Germans held some of the most important economic areas of the Soviet Union, mainly in Ukraine. However, the Germans were pushed back from Moscow. 
The German attack opened the Eastern Front, which in Russia is called the "Great Patriotic War". In the following years Axis powers could not do another attack as large and long as Operation Barbarossa.
War continues.
The winter of 1941-1942 began unusually early. It was not noted as being particularly bad by the Soviets, but the Germans were accustomed to warmer temperatures and so were ill-prepared. The Germans' equipment broke down in the cold, and some men were still wearing summer uniforms, which they stuffed with newspapers in an attempt to stay warm. Many German soldiers froze to death in the extreme cold.
Operation Barbarossa was followed in 1942 by the failed "Fall Blau" (Case Blue), which was intended to take the Caucasus region and its oilfields.
A few years later, the Soviets pushed the German soldiers and invaded the German-occupied territories in Eastern Europe.
The Soviets then invaded Germany itself and fought the Battle of Berlin, the final battle before Germany's surrender. Although the Soviets eventually repelled the Germans, the tremendous cost was five times as many Soviet deaths as Germans.
Operation Barbarossa was the largest military operation in human history by the number of men involved and by the number of people who died. The operation opened a theatre in which more men were involved than any other in history.