When the war began, Klein joined the army and served with the infantry on the Western Front. Commissioned in March 1915, he transferred to the German Air Force in the spring of 1916 and was assigned to Jasta 4 in November 1916. Credited with his first victory in the spring of 1917, he was wounded in action on 9 May 1917. He was wounded again at Gistel on the morning of 13 July 1917. On 27 September 1917, Klein assumed command of Jasta 10. After scoring 6 more victories, he was wounded again, losing his right thumb on 19 February 1918. Upon recovering, he rejoined Jasta 10 but served the remainder of the war as a ground officer. In 1935, Klein joined the Luftwaffe, later served as Deputy Commander of all fighters and attained the rank of Major General before his death in 1944.