During the 1960s, Jan Křen became one of the first Czechoslovak historians to document and research the expulsion of the Sudeten Germans from the country at the end of World War II. Křen was originally a member of the Communist Party of Czechoslovakia from 1949 to 1969, but was expelled in 1970 over his opposition to the Warsaw Pact invasion. He was also fired from his job as a professor and was forced to work as a manual laborer.