The Molotov-Ribbentrop Pact (also known as the Nazi-Soviet Non-Aggression Treaty)
Nazi Germany and the Soviet Union signed a Non-Aggression treaty on August 23, 1939. This treaty between Nazi Germany and the Soviet Union set the stage for the German invasion of Poland a week later, which was the opening round of World War Two in Europe. The second part of this treaty remained secret, and called for the division of Eastern Europe between Hitler and Stalin.
By getting Stalin to agree to not oppose an invasion of Poland, Hitler was assured of a fairly easy war, or so he thought. The agreement to divide Eastern Europe between them ensured that Moscow would not join the British and French in a defense of Poland. The Soviet victory four days earlier in the short border war with Japan (See Battle of Khalhkin Gol) allowed Stalin to free up forces for his own aggression against Poland, the Baltic States, and Finland.
Had these two bloody-minded tyrants not signed this treaty, Hitler may not have invaded Poland, thereby triggering the European phase of World War Two.