Yalta Pact
The Yalta Pact split up postwar Europe and was essentially the spark that began the tensions of the Cold War. The pact later became a source of conflict once the Cold War truly began, as it split up Berlin, and was the first real discussion of the Big Three about how whoever imposes upon a territory also imposes their social and political styles.
The Big Three meeting room
The "Big Three" at the Yalta Conference: Winston Churchill, Franklin D. Roosevelt and Joseph Stalin, 1945.