Arthur Koestler (5 September 1905 – 1 March 1983) was a Jewish author and journalist. He was born in Budapest, Austria-Hungary, but later lived in various other countries. In 1931, Koestler joined the Communist Party of Germany until, disillusioned by Stalinism, he resigned in 1938. In 1940, he published his novel Darkness at Noon, claimed to have been one of the most influential anti-Soviet books ever written. Koestler espoused many political causes, including Zionism, and wrote novels, memoirs, biographies and numerous essays.
His 1976 book The Thirteenth Tribe supported the Khazar theory, as discussed in the article on this topic.