Operation Bagration.The Soviet Army under Stalin began offensive Bagration into Poland; in 6 weeks advanced 500 km to the Vistula. Three years to the day after Germany's 1941 invasion of the Soviet Union, the Red Army launched a massive offensive in Byelorussia. Codenamed 'Operation Bagration', this campaign climaxed five weeks later with the Red Army at the gates of Warsaw. The Wehrmacht's Army Group Centre was routed, a total of 17 Wehrmacht divisions were utterly destroyed, and over 50 other German divisions were shattered. It was the most calamitous defeat of the German armed forces in World War II.
And few in the West has even heard of it considering the fact that D-Day was chicken feed compared to Operation Bagration.
GRAPHIC ACCOUNT OF THE WAR: DECIMATION OF THE THIRD PANZER ARMY