ELI10 why do countries cozy up with Saudi Arabia if they are such a destabilizing force?
Their wahhabism export has already fked up my country big time.
Who do you think takes over if House of Saud falls?
Rewinding a little bit, Saudi Arabia has always been US' ally since breaking away from Ottoman Empire, and it was a dirt poor, extremely conservative, tribal nation ruled by clans and legacy of Abdul Wahhab was always a big force. For the uninitiated, Abdul Wahhab was a renegade preacher who rebelled against Ottoman empire in the 1700s, and was jailed/exiled multiple times by the authorities. The Ibn Sauds saw him as a lightning rod that can be used to polarize and gain support against the Ottomans, which they did. A rebellion that led to the first Saudi State and later on the second Saudi State which were both subsequently crushed by the Turks. Long story short, The Ibn Sauds used Abdul Wahhab in the past to gain support among the deeply conservative society in the past, and today they do the same with As-Sheikh family (Grand Mufti of Mecca). They do this in order to form cohesion and support among the conservative elements of the citizenry.
These conservative elements have tried to take down the Saud dynasty in the past, with the most dramatic fashion during the 1979 Siege of Grand Mosque led by the firebrand Juhaiman Al Othaibi. Al Othaibi and his renegades actually occupied the Grand Mosque in Mecca, killed pilgrims and demanded the Sauds step down because they were too liberal (being very generous overall here, as the renegades had more demands). This is what we face with if the House of Saud falls. The alternative is extremely worrisome.
We allied with the Sauds further during the Soviet Union's occupation of Afghanisthan. After the Oil boom, some people (who were still religiously conservative) became overnight millionaires and billionaires - such as the Bin Ladens. Many Saudis including the government sent millions of dollars to Afghanistan to prop the Mujahideen to fight the Soviets and of course US was heavily supportive and involved in the effort. Many Saudis themselves went to fight Jihad against the Soviets including a very well known person. This was the beginning of "export" of wahhabism. The money funneled went to the Mujahideen first and after the Soviets were defeated, to the neighboring parts as well so Afghanistan could establish itself. Sadly the money went to waste and towards local warlords like Mullah Omar in order to establish an Afghanisthan state. Mullah Omar goes on to establish the rule of Taliban whereas Osama Bin Laden (who was still supported by many in the kingdom for defeating the Soviets) turned his attention towards USA. We know the history from there.
There are extremely dangerous people who can take over Saudi Arabia if House of Saud falls. The government now is experiencing the blowback from all these elements - classic case of chickens coming home to roost. ISIS now would like to take down the Kingdom, and their supporters are in numbers inside the Kingdom, which has been engaged in battles with them (and Al Qaida before them) for almost 15 years now, beginning with the attack on US Embassy in Jeddah back in 2004 I believe. The question leads to "Is the government of KSA supporting ISIS" and the answer is that they did during the uprising against Bashar al-Assad but ever since ISIS established themselves as a "Caliphate", they no longer did. Which is not surprising, as US also supported every rebel group allied against Al-Assad. Currently, US and KSA share secrets and their intel communities work together on battling ISIS.
I may have missed quite a few things but this is as brief of an explanation possible into why US supports KSA.