He was born in Warsaw as Szmul Ostwind son of Wolf (Władysław) Ostwind and Rebeka (Regina) née Saudel. Between 1915 and 1917 he fought in the Polish Legions, in the Polish 1st Legions Infantry Regiment, led by Major Edward Śmigły-Rydz. He participated in the Battle of Kostiuchnówka, among others. After the Oath crisis, he was interned by Germans in Szczypiorno. In 1919 he graduated from military academy as a master sergeant, after which he was assigned to 36th Infantry Regiment, and participated in Polish–Soviet War of 1920. In the interwar period he was baptised into the Catholic Church, and worked for the state police.