26 April 1920 saw the end of the San Remo Conference, at which agreement was reached by the League of Nations on how to divide up the Ottoman Empire's former territories in the Middle East. The League of Nations set into international law a Jewish state in Palestine as well as Arab states in Lebanon, Syria and Mesopotamia. At the Treaty of Sèvres in August 1920 Turkey agreed to relinquish its claim over those territories, although it was not until the Treaty of Lausanne in 1923 that other aspects of the former Ottoman Empire were resolved and Turkey pledged not to seek to re-conquer any of its former territory.
In July 1922, the League of Nations ratified the British Mandate of Palestine as the territory shown as pink in the map at the top of the page. However, international politics is never simple. In 1921, the UK had established the Emirate of Transjordan comprising 77% of Mandatory Palestine and in 1923 unilaterally declared Transjordan to be a state under the control of Emir (later king) Abdullah. A series of concessions to Arab nationalists led up to the White Paper of 1939 which would restrict Jews' ability to purchase land to only 5% of Palestine, whilst also severely restricting Jewish immigration for a period of five years after which it would end completely. During the Mandate period, the British authorities permitted unrestricted Arab immigration to Palestine whilst severely restricting the ability of Jews to immigrate.
In 1945, the United Nations was created and Article 80 of the UN charter provided that "nothing... shall be constructed... to alter... the terms of existing international instruments." In other words, the law that was created at San Remo still continued to apply: the Jewish people have a legal right to their homeland in what was Mandatory Palestine just as much as Lebanon, Syria and Iraq are entitled to their independence.
The following year, Transjordan became independent of the UK in a treaty signed in London and changed its name to the Hashemite Kingdom of Jordan, under the rule of King Abdullah I. This treaty guaranteed continued military cooperation between the UK and Jordan and this crucially led to the Jordanian Arab League's successes during Israel's war of independence in conquering what became known as the West Bank as well as parts of Jerusalem including the Old City. The Jordanian army which invaded Israel in 1948 was led by 37 British officers and 25 British NCOs: the UK and Pakistan were the only two countries in the world which recognised Jordan's occupation and, from 1950, annexation of the West Bank as legitimate.
It is interesting to speculate what might have happened, had the UK implemented the San Remo agreement and created a Jewish state somewhere in the 1920s or 1930s. It is difficult to imagine that the Holocaust could have happened: at least, European Jews would have had the opportunity of a safe haven. For the Jews to be a minority yet preserving a distinctively Jewish state is clearly an issue for democracy looking at it through modern eyes, but that was not a consideration at San Remo. After all, the UK itself did not have full universal suffrage until 1928.