On 20 November 1913, he was promoted to "Oberleutnant" (first lieutenant) and was appointed as the adjutant to the commander of the 80th Field Artillery Regiment. The regiment was deployed to Kolmar in Alsace-Lorraine and at the outbreak of World War I they saw action along the eastern border of Germany known as the Battle of the Frontiers. The 39th Division then participated in the Race to the Sea between September and November 1914, which brought the 39th Division to a position along the Yser River in Flanders, Belgium. While on this part of the front, the 39th Division fought in the Battle of the Yser. In April 1915, Bader was promoted to "Hauptmann" (captain) and was appointed as the adjutant to the commander of the 39th Field Artillery Brigade. After deploying to northeastern France in early 1916 the division reinforced the German attack at Verdun. During the remainder of the war, Bader remained with the 39th Division, which fought in the Battle of the Somme, the Second Battle of the Aisne, the Battle of Passchendaele, and the German spring offensive of 1918.