Ehh, it was mostly the USSR
Eh, it was both. Being trapped between two active fronts caused Germany to split its forces. You close one front up and it's a whole different war.
Attacking the Soviets was his biggest blunder. He should have consolidated what we had and filled in the holes. Like Sweden. Then when your control is cemented you gobble up Spain and Portugal.
All schools now teach only in German, all public food is German, public entertainment(cinema, music, festivals), all new construction is German design, split the populace up. Send German families to conquered nations to live. Then after 15 years you look at expanding. When the entire next generation of Europeans (minus Russia and the U.K.)identifies as German.