"Karski") represented the KPRP at the 1st Congress of the Comintern in March 1919. The KPRP adhered closely to the policies of Rosa Luxemburg. In early 1919, Reiss joined the newly formed Polish Communist Party (the Communist Workers' Party of Poland or KPRP), since his hometown had become part of the Second Polish Republic.
Reiss received the Order of the Red Banner (here, first variant, on red cloth (1918–1924)) He earned a degree from the Faculty of Law, University of Vienna. Reiss also visited Leipzig, Germany, to meet German Socialists: there, he met Gertrude Schildbach, who later aided his assassination. The name Krusia (also "Kruzia") became a codename between these friends in later years. During World War I, the friends traveled when they could to Vienna, where they gathered around Fedia and his girlfriend Krusia. These boys included Kalyniak, Willy Stahl, Berchtold Umansky ("Brun"), his brother Mikhail Umansky ("Misha," later "Ilk"), Fedia (later "Fedin"), and the young Walter Krivitsky (born Samuel Ginsberg). There, he formed lifelong friendships with several other boys, all of whom would become committed Communist spies. His father had his elder brother and him educated in Lwow (modern Lviv), the provincial capital. His mother was a Lithuanian Jew from across the river and his father was non-Jewish.
Reiss was born Nathan Markovich Poreckij in 1899 in Podwołoczyska (today Pidvolochysk), then in Galicia, Austria-Hungary, just across the river from Volochysk, then in Podolia, Tsarist Russia (now both in Ukraine). Reiss' brother died in the Polish-Soviet War in 1920 (here, Polish soldiers display captured Soviet battle flags after the Battle of Warsaw)