The German invasion of the Soviet Union was stopped with:

The Battle of Stalingrad

Explanation:

The Battle of Stalingrad was an armed conflict between the Red Army of the Soviet Union and the Wehrmacht of Nazi Germany, in the context of the German invasion of the USSR during World War II, for the control of the Soviet city of Stalingrad. With more than 2 million casualties in total, this battle is considered the bloodiest battle in the history of humanity. It lasted six months, from August 1942 until February 1943. The grave defeat suffered by the Nazi army in this battle was a turning point for the posterior development of the war and it represents the beginning of the end of Nazism in Europe. The Wehrmacht would never fully recover from this defeat and it would never accomplish another strategical victory in the Eastern front.