"Saudi Arabia" used to just be "Arabia." Then they found oil there, and FDR/Truman promised a local warlord (one of many) named Ibn Saud that he would be recognized as ruler of the entire peninsula if he committed to letting the United States, and not the Soviet Union, purchase that oil.

Saud agreed, as long as the US promised to not interfere with Saud imposing Wahhabism on the rest of the populace there.