Which pact of 1939 secretly divided Eastern Europe?

The Molotov-Ribbentrop Pact, signed in August 1939 by two states that had spent the decade denouncing each other as the ultimate enemy - and the public document was a non-aggression treaty. The part that mattered was a secret protocol dividing Poland, the Baltic states, Finland and Bessarabia into spheres of influence.

Germany invaded Poland a week later; the Soviet Union invaded from the east sixteen days after that, and the two armies met and held a joint parade at Brest. The Soviet Union then annexed the Baltic states and attacked Finland. It also supplied Germany with enormous quantities of oil, grain and raw materials right up until the morning Germany invaded it in 1941.

The secret protocol was denied by the Soviet Union for fifty years. It was published from captured German copies after the war and dismissed as a forgery; the Soviet original was only acknowledged in 1989, after two million people formed a human chain across the three Baltic republics on the fiftieth anniversary to demand it. Confirming a document's existence is not usually a revolutionary act, and in that case it was.