It goes back further than that, all the way back to 1940's and the meeting between FDR and the King of Saud:

https://img.washingtonpost.com/wp-apps/imrs.php?src=https://...

> They eventually came to an agreement that centered around U.S. support and military training for Saudi Arabia, then a fledgling country surrounded by stronger nations, in return for oil and political support in the region.

https://www.washingtonpost.com/news/worldviews/wp/2015/01/27...