Soviet policy following their 1939 invasion of Poland in World War II was ruthless, and sometimes coordinated with the Nazis (see: Gestapo-NKVD Conferences). Elements of ethnic cleansing included Soviet mass executions of Polish prisoners of war in the Katyn Massacre and at other sites, and the exile of up to 1.5 million Polish citizens, including the intelligentsia, academics, priests and Jewish Poles to forced-labor camps in Siberia.