[freshm087]

When you say allies, what comes to my mind is US-UK or US-Israel relationship.

It's quite a narrow view, I dare to say. There's no need any for any stable special relationship, or even shared objectives to form an alliance. Expecting some sort of positive returns is quite enough. E.g. since Germans attacked USSR the latter became ally of UK, and then US in spite of the fact that Soviet ideology made it a natural enemy to both (and predictably USSR became an enemy once Germany lost).

There was an addendum to Molotov-Ribbentrop pact (Secret protocol) which was a plan to divide Eastern Europe between two empires, and it means Soviet-Nazi allyship in 1939 was neither an accident, nor a forced decision.

Well, Nazi plans to expand into the east were well known.

It's not correct in the context. While wish to expand Reich was certainly known, Hitler didn't publicly advertize any clear plans regarding territories under Soviet rule. And Stalin obviously trusted Hitler because he personally discarded intelligence indicating German war preparations as ridiculous. Soviet press in line with party directives kept neutral, or friendly tone towards Germans till the moment of the invasion.