Well, initially the plan was to have the Sharif of Makkah take control of the holy sites after the war. The Wahhabi chieftain ibn Saud was a staunch enemy of the Hashemite Sharif and considered him to be a kafir.
The British agent Abdullah Philby, who allegedly converted to wahhabism, was key in directing ibn Saud not to attack the Hashemites right away, but instead to focus his attention on the Shammari Emir of Hael of the Al Rashid clan, who was allied with the Ottomans, who were in turn of course allied with Germany.
After ibn Saud defeated the Al Rashid and killed their Emir, (and had a son, who would be the late King Abdullah, with the Emir's widow), he turned his attention on the Hejaz.
His advisor, Philby, was instrumental in convincing the British to break with policy regarding the Hashemites and instead to throw their support behind ibn Saud, arguing that he had real control of the hinterlands anyhow, and that a unified Arabia from coast to coast would be in everyone's best interest.
Ibn Saud was given instruction on how far he could go without incurring the wrath of the British. Aden was off limits due to the strategic ports, as were the Trucial Emirates that have since become the UAE, and Iraq and Syria to the north as those were being carved up among the victors of the war. The rest of the desert was fair game and he and his gang of Ikhwan zealots went about marauding and conquering their way through the tribes.
It was said that ibn Saud had two swords, one he fought with and the other he reconciled with. The latter, of course, was a euphemism for his habit of impregnating the daughters of conquered tribes with his future heirs, earning their support.
Once he had no more need for the ikhwan fanatics, he turned on them with modern weapons and destroyed them, claiming the entire kingdom for himself.
When I lived in the UAE, I had conversations with people whose parents and grandparents lived through the Wahhabi invasions of the desert. Apparently people lived in constant fear of a surprise attack the way Europe might have feared the Norsemen.
And for good reason, as well. Just like ISIS, when the Wahhabis attacked, they destroyed ancient shrines, captured women, and killed anyone who disagreed with them.
So, yeah, maybe we should just give daesh another hundred years.
Topic: Fuck Saudi Arabia