Nazi-Soviet Pact (Ribbentrop - Molotov) predates WW2 by a week (German and soviet cooperation goes few years back, they have trained their armies together).
Due to this pact Hitler obtained Soviet help in attack on Poland (Germany attacked on 1 Sept, and USSR on 17th Sept). During WW2 6,5 milion of Polish citizens died in battles, mass murders, roundups, concentration camps, gulags, (17% of population) not counting Jews, Gypsies who also died on Polish soil. I think those numbers are significant.
Due to this pact, USSR was surprised by German's attack in June 1941, and it explains why german army advanced so fast, during first months of the campaign (Stalin didn't believe that such similar soul as Hitler could do such thing to him).
