For many hundreds of years, the Sultans of Turkey had been recognized as the Caliph's of the Muslim world (the spiritual leaders of the Islamic community). On 1st November, 1922, Kemal Ataturk, leader of the Turkish nationalists, abolished the sultanate. Early in 1924, the caliphate was abolished by the Turkish Grand National Assembly.
This left a vacuum which the Shareef of Makkah decided should be filled.
With a dangerous mixture of arrogance and folly, Husain bin Ali proclaimed himself Caliph. The caliphate was not an office to be assumed by an individual, least of all by one who was almost alone in the belief that he was worthy of the title.
While many Muslims saw the Shareef's act as entirely inappropriate and rather pathetic, to Ibn Saud and his followers it was an outrage.