On 19 July 1914, Ungern joined frontline forces as part of the second-turn 34th Regiment of Cossack troops stationed on the Austro-Hungarian frontier in Galicia. He took part in the Russian offensive in East Prussia. From 1915 to 1916, he also participated in rear-action raids on German troops by the L.N. Punin Cavalry Special Task Force. Ungern served in the Nerchinsk Regiment.