After the outbreak of World War I he led his division into the Loraine region on the Western Front. On 9 October 1914, he became commander of the I. Army Corps in Lithuania, where his troops opposed numerically superior Russian forces. After initial tactical setbacks Kosch and his men finally won the Second Battle of the Masurian Lakes. For this victory Kosch was awarded the Pour le Mérite. On 11 June 1915 he was appointed Commanding General of the X. Reserve Corps. He led them in the battles on the Dniester, the Battle of Gnila Lipa and at Krasnostaw. He transferred to the Balkans Theater, where he fought against Serbia. For his success Kosch received, on 27 November 1915, the Oak Leaves to his Pour le Mérite.