British Mandate of Palestine: 25 April 1920 (League of Nations approved 24 July 1922) to 15 May 1948
Before the British occupation, Palestine was part of Ottoman Syria. The British army ruled Palestine until a civil administration was established on 1 July 1920. Britain was granted a Mandate for Palestine on 25 April 1920 at the San Remo Conference, and, on 24 July 1922, this mandate was approved by the League of Nations.
The British determined the borders of Palestine according to other agreements they had made with their allies. For example, they transferred the eastern bank of the Jordan River to the control of Abdullah ibn Husayn (1882-1951) of the Hashemite dynasty and appointed him King of Jordan in recognition of his support during the war.
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https://encyclopedia.1914-1918-online.net/article/british_mandate_for_palestine