[2] In 1910 he began studying law and philosophy at the Jagiellonian University in Kraków, and two years later joined the paramilitary Riflemen's Association.
From 1917, he was an active member of the Polish Military Organisation, and in late autumn of 1918, he participated in the disarmament of Austrian troops in Kraków.
With this unit, he took part in the annexation of Trans-Olza (Český Těšín) in 1938 (see Independent Operational Group Silesia) and was promoted to brigadier general in March 1939.
During the Polish September Campaign, Kustroń's division was part of the Kraków Army, and was engaged in heavy fighting with the advancing Wehrmacht from the first day of the war.
Trying to break through German lines and reach Lwów, the Poles fought a battle with Wehrmacht's 45th Infantry Division.