In order to enlist the military and political support of the Arabs, Britain promises to support their struggle for independence in most of the lands hitherto ruled by the Ottoman Turks, presumably including Palestine (see the correspondence between Sharif Husayn and MacMahon).
At the same time, Britain agrees with France and Russia to carve up the Middle East into mutually agreed spheres of economic and political influence. The map drawn up in the Sykes-Picot agreement contradicts the promises made in the MacMahon correspondence.
Some authors charge Britain with outright duplicity, others are more forgiving, believing that the British later did their best to stabilize the tense situation they themselves had helped to create during the First World War. With respect to Palestine in particular, the Foreign Secretary Lord Balfour conceded as early as 1919 that

the Powers had made no statement of fact that is not admittedly wrong, and no declaration of policy which, at least in the letter, they have not always intended to violate. (Armstrong, p. 374, quoting from Christopher Sykes, Crossroads to Israel, London 1965, pp. 16-17)

Be that as it may, it is clear that Britain's promises could not all be fulfilled and that the mandatory power proved unable to control the flames of nationalism it had nourished.

Image: General Allenby and Emir Abdullah (1920)

Source: Passia

Period
J'lem in early Jewish and Arab nationalism
Zionism and the Jerusalem Question
Location
Link to Map Index
Syces-Picot (map)
Palestine in Sykes-Picot (map)
San Remo (1920-22), map
Palestine and Transjordan after 1922 (map)
Political History
MacMahon's Letter (1915)
Sykes-Picot (1916)
Balfour Declaration (1917)
Sources
"The 27 Articles of T.E.Lawrence"
Lawrence of Arabia
WWI: Britain Makes Promises
Main Jerusalem Timeline > 20th Century > Mandate > Promises