Question

Why Hitler's attack on Soviet Union is in 1941 regarded a historic blunder

Answer 1

During 1941, Germany was already fighting the allied forces on the western front of it's empire. Attack on Russia led to fighting between the two powers on the eastern front of the German empire. In this way, It got 'cornered' from both sides which eventually led to it's downfall.

Answer 2

The historic blunder Hitler committed was that he advanced onto the USSR which left the western and  eastern front of Germany vulnerable to the British forces and the Soviet forces respectively. This let the Soviets attack them and the British guards aerial bomb the area.
This is what led to the defeat of the German Army.