Prior to the Arab-Israeli War in 1948, important promises were made to both the Arabs and the Israelis. In order to receive Arab backing against the Ottoman Empire in WWI, the MacMahon-Hussein correspondence in 1915 mapped out the land that Britain was to cede to the Arabs upon allied victory. In 1917, however, the British also promised to make Palestine the national home for the Jewish people in the Balfour Declaration, possibly to coerce the Jewish lobby in the US to back Britain in WWI. .
Post-WWI Britain received Palestine as a mandate, and soon realized the problem of its earlier equal-obligation to both the Jewish and the Arabs. In 1922 Britain partitioned historic Palestine into two new areas, Transjordan and Palestine. Transjordan was 80% of historic Palestine, and the other 20% wass to be the national homeland for the Jewish people. (Bard 84-5) Jews were not allowed to buy land in Transjordan and Arabs were not allowed to buy land in Palestine. Major immigration of Jews began, and Arabs became increasingly concerned that these aliyahs would prevent them from returning to their homeland.
Under the assumption that the British seemed to support the Arabs, Jews put together Haganah, an underground Jewish militia. They began training and producing arms, all without Britain's knowledge. From Haganah came Irgun, another Jewish militia, this one more radicalized. Many attacks occurred, primarily against British officials, and in 1947, Britain brought the problem to the UN. After investigations, UN decided that Palestine would be further partitioned into two states, one for the Jews and one for the Palestinians Arabs. The UN also concluded that Britain would have to leave by August 1, 1948. Unhappy with the decision and frustrated by the guerilla warfare of the Jews on British officials, Britain evacuated on March 13, 1948. On March 14, 1948, Israel declared independence and on March15, 1948 five Arab armies denounced the plan for further partition and attacked Israel, which began the war of 1948.