On June 22 1941 the forces of Nazi Germany attacked the Soviet Union. This was the largest land campaign in history and casualties on both sides number in the tens of millions.
Some interesting aspects of the German invasion:
1). Was Stalin a fool who refused to believe that Hitler would attack him or was the situation more complex?
2). Were Soviet codes impregnable?
3). Were the German shocked by the performance of the new Soviet tanks like the T-34?
4). Some statistics on German and Soviet losses: