Saudi Arabia (like Kuwait) doesn't have much of a military, but it is absolutely crucial to the world energy market. The US will apparently spare no expense to protect it.
Saudi Arabia has one of the largest and well supported Air Forces in the middle east, other than Israeli AF. The RSAF has 250+ fighter/attack aircraft in service. F-15s and Tornado's mainly. Also another 120 F-5E/F fighter "trainers" available.
That's fine. It would still have been quite vulnerable to a ~1990 Iraq. Even a ~2000 era Iraq wouldn't need to conquer the country, but merely strike critical oil infrastructure. They were thought to have chemical weapons, etc.
If you don't believe the US cares much about such things, I recommend the book (unfortunately titled) "Blood and Oil" by M. Klare.
That and genwin's reason.