Not long after Izso left the opera, World War I erupted. After Archduke Franz Ferdinand of Austria was assassinated, Austria-Hungary declared war one month later and marched into Serbia. Izso was caught up in the quick mobilization and served as a chaplain in the Austro-Hungarian army. He was active for eight months, but was seriously wounded in the leg by an Italian artillery shell fragment during the First Battle of the Isonzo in 1915. He eventually recovered, but not in time to return to service.