The McMahon-Hussein Agreement of October 1915 was accepted by Palestinians as a promise by the British that after World War One, land previously held by the Turks would be returned to the Arab nationals who lived in that land. The McMahon-Hussein Agreement was to greatly complicate Middle East history and seemed to directly clash with the Balfour Declaration of 1917.
In an effort to create a third front against the Central Powers (Germany, Austria and Turkey) the Allies encouraged the Arab people in the Ottoman Empire to rise up against their Turkish overlords thus splitting the Central Powers war effort three ways.
Sir Henry McMahon, acting on behalf of the British government, met with Sherif Hussein of Mecca in 1915 and made what were taken to be a series of promises to the Arab people. These 'promises' were later disputed by the British government and, as with many issues concerning recent Middle East history, were open to interpretation.
Hussein interpreted the correspondence given to him by McMahon as a clear indication that Palestine would be given to the Palestinians once the war had ended. The British government was later to dispute this interpretation. They claimed that any land definitions were only approximate and that a map drawn at the time (but not by McMahon or a member of the British delegation) excluded Palestine from land to be given back to the Arab people.
The confusion arose from one small phrase in the correspondence between McMahon and Hussein. Land that “cannot be said to be purely Arab” was excluded from the agreement - as far as the British were concerned. Hussein, and very many Arab people, considered Palestine to be “purely Arab”. The British saw Palestine differently as the Turks, while they had been masters over Palestine, had allowed other religious groups to exist in Jerusalem - hence their belief that Palestine “cannot be said to be purely Arab”.
By the time war ended in November 1918, two distinct schools of thought had developed regarding Palestine:
1) That the British had promised Palestine to the Arabs after the war had ended in return for their support to the Allies in the war.
2) That the British had agreed to give their support to the Jews for a homeland in Palestine as laid out in the Balfour Declaration of 1917.
In fact, neither was to emerge as the League of Nations had given Palestine to the British to govern as a mandate. This left many Palestinians feeling that they had been betrayed by the British government. At the same time many Jews started to enter Palestine as a result of what they believed the Balfour Declaration had offered them. The British were left to ensure law and order was guaranteed in Palestine - something they found increasingly difficult to do.