In late 1918, Wieczorkiewicz joined the Polish Army, and was immediately promoted to Major. In November 1918, he created a unit of volunteers, which fought in the Battle of Lwow. Since August 15, 1919, he commanded 9th Legions Infantry Regiment, with which he fought in initial stages of the Polish–Soviet War. By the end of this conflict, Wieczorkiewicz commanded 3rd Legions Infantry Division.