Today is the 70th anniversary of the Nazi-Soviet Pact, the infamous agreement in which Nazi Germany and the Soviet Union tried to carve up Eastern Europe between them. Historian Orlando Figes has a good summary of its significance here. History is full of cynical international agreements, many of which led to terrible results. But I doubt anyone can point to a worse treaty than this one.
The agreement set the stage for history's bloodiest war, which killed some 50 million people. Without assurance of Soviet noninterference, the Nazis could not have gone to war against Britain and France (they realized that, in 1939, they lacked the military power to fight a two-front war). The agreement also enabled both powers to inflict horrible atrocities against the people of the Eastern Europe states they occupied as a result. Everyone knows about the Nazi part of these crimes. The Soviet part is less well-known, but almost equally heinous. For example, the treaty gave the Soviets the "right" to occupy the Baltic States, and Eastern Poland. This led to the extermination of some 3% of Estonia's population, and the deportation to Gulags of many more. The other areas occupied by the USSR (including a large part of eastern Poland) suffered comparable atrocities.
It's hard to precisely calculate the overall harm caused by the Nazi-Soviet Pact. But the death toll surely runs into the millions.
Defenders of Stalin's decision to sign the pact claim that he needed to do it because the British and French otherwise might have simply stood aside and let Hitler attack him. It's hard to defend the Anglo-French appeasement of the late 1930s. But at least they did not actively collaborate with Hitler, as Stalin chose to do. Moreover, Hitler could not have attacked the USSR in 1939 without going through Poland, which the British and French had just guaranteed against German attack. Finally, by allowing Hitler to deal with his Western enemies before having to worry about the Soviets, Stalin set up a situation where the Nazis could, in 1941, attack the USSR without having to face any other opponent on in Europe on land. By signing the pact with Hitler, Stalin himself helped create the absence of a "second front" that he later spent much of World War II complaining about.
On a more personal note, my great-uncle was killed in the Russo-Finnish War, just a few months after the pact was signed. Finland was, of course, one of the states allocated to Soviets under the agreement with the Nazis. It is unlikely that Stalin would have dared to attack Finland without first being assured of German noninterference. Thus, my relative became one of the millions who lost their lives as a result of history's most infamous agreement.
The European Parliament has proposed that August 23 be declared a day of remembrance for the victims of totalitarianism. It's definitely an appropriate way to mark the occasion.