The Nazi-Soviet pact, otherwise known by the names of the respective foreign ministers who signed it in 1939, led to the carving up of Poland and other parts of eastern Europe at the start of the Second World War.
It saw Poland virtually disappear from the map, with 20,000 Polish officers and intellectuals executed by the Soviet secret police in the Katyn massacre, and up to three million Jewish people dying in Poland as a result of the Holocaust.
The Nazi-Soviet pact also effectively divided up Estonia, Lithuania, Latvia, and Romania. Joseph Stalin was so shocked by Adolf Hitler's later betrayal of the non-aggression treaty that he initially refused to believe it.
And Vladimir Putin just can't make up his mind about the whole thing.
Meeting with young historians in Moscow this week, the Russian president urged a re-examining of the build-up to the Great Patriotic War and the west's role in Hitler's rise.
He said western historians had tried to "hush up" the 1938 Munich Agreement between Britain and France and Germany, which was the culmination of the appeasement policy pursued by the government of Neville Chamberlain.
On the Molotov-Ribbentrop pact signed a year later, Putin said:
The Soviet Union signed a non-aggression treaty with Germany. People say: 'Ah, that's bad.' But what's bad about that if the Soviet Union didn't want to fight, what's bad about it?
This is far, far from his viewpoint only five years ago, when, as prime minister, he marked the 70th anniversary of the invasion of Poland by writing, in a Polish newspaper no less, that the pact was "immoral".
He went on to write:
It is possible to condemn - and with good reason - the Molotov-Ribbentrop pact concluded in August 1939.
We can perhaps learn a lot about Putin's changing attitude to Europe and its territorial integrity in his apparent change of heart.
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