The USSR and Germany concluded a non-aggression-pact in 1939 with a secret amendment to divide Poland and the Baltic states between themselves. This late became known as the Molotov–Ribbentrop Pact (or Hitler-Stalin-Pact, or Nazi-Soviet-Pact). With the Sowjet occupation of Bukovina in 1940 and definitely the German attack on the USSR ("Operation Barbarossa") in 1941, the pact was defunct.
In December of 1941, after Pearl Harbour, the US entered the war, and then you had the main combatants in place:
The allies with the USSR (Stalin), USA (Roosevelt), British Empire (Churchill), France (de Gaulle), Republic of China (Chiang Kai-Shek), against
The axis with Germany (Hitler), Italy (Mussolini), and Japan (Hirohito).
