In September of 1939, almost exactly 80 years ago, Nazi Germany and the Soviet Union started World War II  by invading Poland.  Though Hitler double-crossed Stalin two years later, the secret provisions of the Molotov-Ribbentrop Pact explicitly divided Eastern Europe between them.  How was this alliance possible – and what was it all about?  False modesty aside, I think that my encyclopedia articles on Communism and Fascism are a fine place to start.