On June 22nd 1941, the Axis forces invaded the USSR. The eastern european front of the WW2 was the deadliest conflict in the history of the mankind.
This video is interesting. It explains the failure of the operation without using cliches like the cold, the millions of suicide soviet soldiers, the "crazy Hitler" who didn't listen to the "competent" generals. According to this video, the nazi underestimated the soviet forces both in numbers and technology, Hitler had a misleading view that Stalin's regime would colapse when the invasion starts, the lack of cooperation between Germany and Japan played a role
https://www.youtube.com/watch?v=A_3R-Rkn_98