During the invasion of Poland in September 1939, Biss commanded a Polish Army company.He was captured by the Germans near Lwów, but after a few days in detention, he escaped. He stayed in eastern Poland, which was annexed by the Soviet Union. Before the German invasion in USSR started in June 1941, Biss worked as a teacher in the Kołomyja area, where he also joined the Polish resistance. In November 1940, he started serving in the Union of Armed Struggle (ZWZ), a precursor of the Home Army, which he subsequently joined.