Zeumer reportedly already had a pilot's license when
World War I broke out in mid-1914. He joined the German flying service in August, and served with
Feldflieger Abteilung 4 (Field Flyer Detachment 4, or FFA 4). By November 1914, he had been awarded the
Iron Cross and the Knight's Cross of the
Military Order of St. Henry. At some point he also received an Ehrenbecher, or "honor cup", From May to August 1915, Zeumer flew on the Eastern Front with Feldflieger Abteilung 69 (FFA 69), where he befriended
Manfred von Richthofen, better known as the Red Baron. Richthofen, then an
observer with no victories to his credit, would go on to become the highest-scoring ace of World War I. Zeumer was Richthofen's first pilot, and in August 1915, the two men flew a
Gotha G.I in
Ostend,
Belgium, as part of the Brieftauben-Ableitung-Ostende (BAO) unit. Zeumer was with Richthofen during the latter's first aerial combat, in which they tried unsuccessfully to down an Allied reconnaissance plane. Richthofen wrote of the experience: Despite this spat, Richthofen and Zeumer had a close relationship, with Richthofen referring to Zeumer as "my good friend" and writing that Zeumer "flew with rare skill." When Richthofen decided to move from being an observer to a pilot, it was Zeumer who taught him how to fly. In early 1916, both men served with bombing unit
Kampfgeschwader 2. Zeumer achieved four confirmed victories there, the first of which was a French Nieuport Scout over Douaumont on 11 April 1916. In June 1916, Zeumer was shot down by the French. Although only slightly injured by the crash, he broke his right thigh in a car accident while being transported out, under what Richthofen described as "quite stupid circumstances." At some point Zeumer was promoted to
Oberleutnant, the highest lieutenant officer rank in the German armed forces. In May 1917, he joined
Jasta Boelcke, a fighting unit founded by famed German ace
Oswald Boelcke. ==Personal life==