Drafted into the Austro-Hungarian Army in July 1914, Rydz was transferred in August to the Polish Legions and fought in World War I in the famous Polish 1st Brigade of Józef Piłsudski. He took part in numerous engagements against the Imperial Russian Army in the region of the Southern Vistula and rose quickly in rank. By 1916, he was already a full colonel. He did not forget his art, however, and exhibited his work at a gallery in Kraków around that time. In 1917, after their refusal to swear an oath to the Austrian and German authorities, the Legions were disbanded, their soldiers interned and their leader Józef Piłsudski imprisoned in Magdeburg fortress. By Piłsudski's appointment, Rydz (who was released from prison on grounds of ill health) became commander of the Polish Military Organization ("POW") and adopted the nom de guerre "Śmigły" ("Fast" or "Agile"), which he later officially added to his surname.