Klemens Stanisław Rudnicki (nom de guerre Klimek) was born on 28 March 1897 in the town of Żydaczów, Austrian Galicia (now Zhydachiv, Ukraine). As a teenager, he belonged to paramilitary Polish organizations, such as Polish Rifle Squads and Zarzewie. In August and September 1914 he served in Legion Wschodni, and then was drafted into the Austro-Hungarian Army. Rudnicki fought in the Battles of the Isonzo, where he was seriously wounded.