During World War I he joined the Polish Legions of Józef Piłsudski as a private. He was promoted to the rank of lieutenant and took part in the Battle of Kostiuchnówka, for which he was awarded the Polish Cross of Valor. During the Oath crisis, when Polish troops refused to swear allegiance to Emperor Wilhelm II of Germany, Lieberman served as the lawyer for the Polish soldiers who were charged with treason by the German authorities.