Stalin. He, along with Adolf Hitler, made a number of historical decisions, which helped impact World War II. Before the War begins, Joseph Stalin and Adolf Hitler signed the German-Soviet Nonaggression Pact. From there, Stalin annexed many parts of the region, including Poland and Latvia. Along with those two decisions, Stalin decided to invade Finland.
However, as the war went on, and the Germans began to invade other regions, Stalin began to attend the conferences of the Allied forces. In the end, Stalin was able to set himself up as being a loyal ally to those that were loyal to him and his country.