On October 31, 1918, Lieutenant Colonel Bortnowski joined the Polish Army which was reborn after Poland had regained her independence at the conclusion of World War I. Initially, he led a company in the 5th Infantry Regiment, and was later promoted to lead a battalion of the 5th Infantry Regiment. In November 1918, he participated in both the Sieges of Przemyśl and Lwów. From May 1919, Bortnowski worked as a Teaching Assistant during the very first post-war sessions at the War School of the General Staff of the War College in Warsaw.