How Jordan Came To Be
The Ottoman Empire ruled Palestine, the land renamed by the Romans, from 1517 to 1917. It was just an empty expanse of land and swamps. There was no government, no country.
Turkey was on the axis side in World War I and lost. As a consequence, she lost all her power. The French and British mandates were then set up with France taking over the area now called Lebanon and Syria. The British got Palestine and Iraq. In 1926 Lebanon was separated from Syria.
Britain installed Emir Faisal, who had been in Syria but was evicted. He was then made ruler of the new kingdom of Iraq.
In 1922, the British created the emirate of Transjordan, which included all of Palestine east of the Jordan River. They did this because Emir Abdullah would have a Kingdom to rule. After all, his family had been defeated in tribal warfare in the Arabian peninsula and lost out.
He may have been the Emir, but it remained under British supervision until after World War Ii. In 1946 the British asked the UN to end their British Mandate. Then in May of 1948, Israel was born into the UN.
King Abdulla was made the first ruler of the Hashemite Kingdom of Transjordan and was assassinated in 1951.
These two nations, Jordan and Israel, were inevitably linked in the President Truman's mind as twin emergent states: one serving the needs of the refugee Jew, the other absorbing recently displaced Palestinian Arabs. In addition, Truman was aware of the private agreements that existed between Jewish Agency leaders and King Abdullah I of Jordan. Thus, it made perfect sense to Truman to favour both states with de jure recognition.
Reference: Myths and Facts by Mitchell G Bard and Joel Himelfarb.