During WWI, Sir Henry McMahon, the high commissioner in Egypt, exchanged ten letters with Sharif Husayn ibn Ali, Emir Faisal's father. There is a vast controversy about what was agreed upon in those letters. Throughout the Husayn-McMahon Correspondence, as it came to be known, Husayn insisted upon control of the Syrian coast. McMahon refused it repeatedly, but a decision on the matter was ultimately postponed. After a long correspondence, McMahon ultimately promised that "Great Britain is prepared to recognize and uphold the independence of the Arabs in all the regions lying within the frontiers proposed by the Sharif of Mecca."^(1) They also promised to provide materiel, funds, and other supplies if he would revolt against the Ottomans. Husayn came away with the understanding that Britain would support his claim as ruler of an independent Arab state, but Britain had a different idea in mind. This misunderstanding is what leads to Faisal becoming King of Iraq rather than Syria.
Meanwhile, the British and French had agreed to split up the Ottoman Empire among themselves (and Russia) in the Sykes-Picot Agreement of 1916. It seemingly directly contradicted Britain's promises to Husayn by awarding France direct control over the Syrian coast and a zone of influence over the Syrian interior. In exchange, Britain was given direct control over Iraq and influence over an area including Palestine and Transjordan. This whole mess was further complicated by Lord Balfour's Declaration in 1917, but that is a different subject.
After the war, the French and British carved up the remaining provinces of the Ottoman Empire. Britain created the countries of Iraq and Transjordan and France took control over its new Syrian possession. While these negotiations were taking place, Faisal was working to form his own Arab government in Damascus. In 1920, Faisal's new government met together in the Syrian congress and declared that Syria was an independent state, and Faisal was its king. France was predictably displeased and put pressure on Britain to renege on its promises to Sharif Husayn's family. Britain caved and renounced its support of Faisal. France easily swept Faisal's forces aside on July 24, 1920, and occupied Damascus. Faisal's fledgling kingdom lasted only five months.^(2) In the aftermath of this debacle, Britain offered the kingdom of Transjordan to Husayn's other son: Abdullah. Husayn remained in Mecca.
Britain had a vital interest in the new country of Iraq. They needed control over Iraq to secure their lines of communication to India and the Iraqi and Iranian oil fields. They ultimately gave the rule of Iraq to Faisal. Whatever their reasons for providing Faisal with a second chance, Faisal became King of Iraq in 1921. He had his work cut out for him, as Iraq was an amalgamation of three separate Ottoman provinces and was not entirely Arab. He became king of a country he did not want and over a people that did not want him. It was a doomed prospect from the beginning.
^(1)Sir Henry McMahon to Sharif Husayn, October 24, 1915, in J.C. Hurewitz, ed., The Middle East and North Africa in World Politics: A Documentary Record, vol. 2, British French Supremacy, 1914-1945, 2nd ed., p. 50
^(2)A History of the Modern Middle East, 5 ed. pp.144-154
Not cited: A Peace to End All Peace, David Fromkin