Russia was just coming out of a Revolt and was still having to deal with things like that, Though it depends on which date you are referring to, the years of 1934-6 when the Japs started going hog wild in Asia, our 1939 when Hitler started going hog wild in Europe, The date would help.
It would be interesting America was coming out of a depression but Russia was still rebuilding from a Revolt in the 30's. So to be honest America probably was the weaker of the two. However most likly if it was the 34-6 The Euro Powers would have stayed out, however after 39 and the Russian/German Invasion of Poland Britian and them would have most likly help America fight off the Russians.