This week marks the 80th anniversary of the invasion of Poland by the USSR on September 17, 1939. Coming just sixteen days after the Nazis attacked from the West, the Soviet invasion was part of a secret agreement, the Molotov-Ribbentrop pact, in which Germany and Russia agreed to split Poland's territory before wiping it off the map of Europe. Listen to learn more about this fascinating but lesser-known chapter of the Second World War.