What might be considered a reason for germany's inability to maintain alliances with russia?

Assuming you are talking about the second world war Germany formed an alliance of sorts with Russia to ensure safety on its eastern border (a lesson learnt from the first world war). After Germany had captured France they were then able to turn to Russia (Germany had no intent of remaining allied with Russia). At the time the Germans considered the Russians inhuman like everyone else and therefore should be destroyed.

If this isn't the second world war please tell me.