"During World War I, most Jews supported the Germans because they were fighting the Russians who were regarded as the Jews' main enemy. In Britain, the government sought Jewish support for the war effort for a variety of reasons including an erroneous antisemitic perception of "Jewish power" over the Ottoman Empire's Young Turks movement, and a desire to secure American Jewish support for US intervention on Britain's behalf.
There was already sympathy for the aims of Zionism in the British government, including the Prime-Minister Lloyd-George. In late 1917, as the British Army (including a mainly Zionist Jewish Legion) drove the Turks out of Palestine, the British foreign minister, Lord Balfour sent a letter to Lord Rothschild. The letter subsequently became known as the Balfour Declaration of 1917. It stated that the British Government "view[ed] with favour the establishment in Palestine of a national home for the Jewish people".
In 1918 Chaim Weizmann, president of the British Zionist Federation, formed a Zionist Commission, which went to Palestine to promote Zionist objectives there.
The combination of Jewish immigration and the terms of the Mandate led to Arab rioting in 1920 and 1921. In response, the British authorities enacted a system of immigration quotas. Exceptions were made for Jews with over 1,000 pounds in cash (roughly 100,000 pounds at year 2000 rates), or Jewish professionals with over 500 pounds. Arab attacks on isolated Jewish settlements and the British failure to protect them led to the creation of the Haganah ("Defense"), a mainly socialist underground Jewish militia dedicated to defending Jewish settlements."
