Jäger served on the Russian front with Infantry Regiment No. 67. Wounded for the first time on 30 August 1914, he recovered and returned to duty only to be wounded in the chest on 21 March 1915. Again he returned to duty and again he was wounded. This time he was shot through the lung on 17 May 1915. When he recovered he served as a training officer in Hungary and by the following year he applied for and was accepted into the air service. In the spring of 1916, Jäger was posted to the Russian front with Flik 10 as an observer. Having scored five victories, he trained to become a pilot during the last three months of 1916 and was posted to Flik 17 in March 1917. On 2 May 1917, Jäger was injured when a revolving propeller struck his legs. He recovered and served briefly with Flik 3 on the Russian front before being reassigned to Flik 27 in July 1917. During the Kerensky Offensive that month, he scored his sixth victory and was reassigned to Flik 42J in August 1917. Shortly after scoring his last victory on the morning of 19 August 1917, Jäger was killed in action when his Albatros D.III (153.14) was shot down by an Italian Nieport. He was buried with full military honors and posthumously awarded the Order of the Iron Crown, 3rd class, with swords.