Historical Text on the Origins of WW2 on the Eastern Front - Peer Review Requested

[Franek Grabowski]

Well, would say the text is simply incomplete. It is not the problem of writing a thesis on the subject, but rereading it and introducing changes.
Say eg. your sentence.

Despite the non-aggression pact – proposed by the signing parties to be valid for ten years – signed by the foreign ministers of the USSR and the IIIrd Reich – Vyacheslav M. Molotov and Joachim von Ribbentrop, respectively – on 23 August 1939, pact that surprised many, as well as divergent diplomatic moves, both sides actually prepared fervently to attack the other.

Could be replaced with a better effect (IMHO) ina following or similar manner.

Despite signing a pact of mutual assistance and non-aggression, that resulted in strict military, intelligence and economical cooperation and partition of Eastern Europe, both Germany and Soviet Union found themselves conspiring against each other.