Not sure how the entire Nazi comparison plays out, but at the time there were good reasons to imagine the Soviets will fall apart (as they initially did)

Stalin just finished purging his entire officer corps, which is not a good omen for war, and the USSR failed miserably against the Finnish who were not the strongest of nations, while Germany just steamrolled France, a country that was much more impressive in WW1 than the Russians (who collapsed against Germany)