After the outbreak of the First World War, on August 20, 1914, he would join the Polish Legions. He would serve in the 3rd Brigade, 3rd and 4th infantry regiments. He would take part in the Carpathian campaign and the Battle of Mołotków and during this time he would graduate from cadet school. He would be seriously wounded on 31 August 1915 in the Battle of Jastków. Later he would be promoted to the rank of ensign and then he would be promoted again to second lieutenant and he would take part in the Volhynian campaign. He was wounded again at the Battle of Rudka Miryńska in 1916. During the Oath Crisis of 1917 he would spend 4 days in jail. He would also later be active in the Polish Military Organisation.