Imagine if Iran were to invade Saudi Arabia which is not a farfetched scenario at all, in fact this is precisely what Tehran plans to do, what would the United States do? Iranian control over GCC oil wells would overnight transform Tehran into a superpower with the ability to extort US allies around the world through oil embargoes. The US would literally have no choice but to deploy a vast force to liberate the Arabian peninsula from its Iranian occupiers. Considering that the size of the Saudi military is so incomparably smaller than the Iranian military is it questionable how long the Saudis would be able to hold out without American intervention. The other Gulf states simply cannot defend themselves. Therefore a US-Saudi defense pact is simply a no-brainer. However, it is in the interest of the United States to prevent war rather than engage in war and a US-Saudi defense pact is simply the single most effective way to prevent war between Iran and the KSA and thus prevent US entanglement in the Middle East.