Erm, Stalin did a deal with Hitler, the Molotov–Ribbentrop Pact. In a secret protocol, they agreed to carve up Europe between them with the USSR to take Lithuania, Latvia, Estonia, Finland, Bessarabia and half of Poland. The Finland part didn't work out so well, and Hitler went on to break the pact. The details of this were proven only when it was made public during the Nuremberg trials.