This was the 1939 agreement between Hitler (Germany) and Stalin (Russia) where they agreed to non-aggression and to divide Poland between them. (It was named for the Foreign Ministers of both countries.)
The areas of Poland that were invaded ended up with Russia after the war, and are now in Belarus and Ukraine.
Germany would renege on the agremeent in 1941 when it invaded Russia.
Why I Looked It Up
On an episode of The Orville, when Captain Mercer sees that the Krill and Moclans have formed an alliance, he remarks:
It’s like Molotov-Ribbentrop all over again.