After the outbreak of World War II with the German invasion of Poland, the Soviet Union also invaded on September 17, as part of the Molotov-Ribbentrop Pact between Nazi Germany and Soviet Union. Feiner was caught in the Soviet part of occupied Poland, was arrested by the NKVD and spent several months in a Soviet prison in Lida, near Wilno. Despite the fact that before the war on several occasions he had defended Polish Communists in court as an attorney, and that he had belonged to a socialist organization (the General Jewish Labour Bund in Poland), the Soviets authorities charged him with being a "fascist" and a "counter revolutionary" After the Nazi invasion of the Soviet Union Feiner managed to escape after the Soviets fled Lida in 1941, and made his way to Nazi-occupied Warsaw.