From November 1918 Fieldorf served in the ranks of the Polish Army in the newly forming Second Republic, initially as a platoon commander, and from March 1919 commanded a heavy machine gun company. In 1919 and 1920 he took part in the campaign to join the Wilno region to Poland proper. After the commencement of the Polish-Bolshevik War, as a company commander he participated in liberating Dyneburg, Żytomierz and in the 1920 Polish Expedition to Kiev.