The promise was that Israel would be created where it had originally been. At this time there were about 85,000 to 100,000 Jews living in Palestine where the land of Israel had been. Most of the Arabs living there had migrated in the past 30 years because they were attracted by jobs offered by Jews who were building and farming. Jews had immigrated in large numbers in 1882 when only 250,000 Arabs were living there. Arabs mostly were a migratory people.
In 1917 Earl Arthur Balfour, foreign secretary, promised British support with the Balfour Doctrine. Previously in 1906, they had a conversation about an offer he had made in 1903 to take Uganda instead of Israel, even though at that time the Ottomans still controlled the Middle East.
Weizmann asked Balfour, "Would you take Paris over London?
Balfour replied, "But we already have London". He meant that Jews should take whatever they could get, and that beggers couldn't be choosers.
Weizmann retorted back, "Mr. Balfour, the Jews had Jerusalem when London was a marsh."
What Balfour didn't know was that we all had made a promise ever since the fall of Jerusalem in 70AD by the Roman attack was that we would not forget it, lest we would lose our right hand. Being of this generation, I haven't forgotten and neither have my peers. Uganda? Not a chance. If Americans suddenly lost the USA, I wonder how long future generations would remember and crave Washington DC?
Balfour's promise was watered down so badly that one can hardly recognize it. We did not receive the parcel of land promised, but just a piece of the pie. Other Arab rulers were able to feather their nests around 1932 and get nations like Iraq and Saudi Arabia. We had to wait until May 1948, and even at that had to fight the minute we announced our statehood to keep it. So much for broken promises.