Let's say that Germany decides in early 1941 to go with Op Barbarossa despite the USA entering the ww2 in 1940.
They need to deal with Yugoslavia and Greece, help Italians not to loose in Libia, defeat Soviet Union, keep a knife under Vichy's throat so those don't chage sides, keep a sizable garrison from north of Norway down to Spanish border, all while avoiding not to be defeated by RAF and USAAF in mid-1940. Quite a tall order, don't you think?
It would be, but in this scenario the swift conquest of the USSR is even more necessary however difficult. Germany needs to secure the resources to prosecute the war and remove one major enemy. The idea of fully drawing the USSR into the Axis is superficially attractive but even setting aside the ideological issues the Germans understood that they would rapidly be reduced to the status of junior partner, in the same way that Britain found itself beholden to the USA. To be clear Barbarossa in 1940 will almost certainly fail, but the USSR is at the nadir of its military effectiveness, so there is a slender chance of success.