In 1915 the British government and Ibn Saud signed an agreement which "made the lands of the House of Saud a British protectorate". Why isn't Saudi Arabia usually depicted on maps as being part of the British Empire at this time, as is the case with the other arabian British protectorates? Was the protectorate aspect of the agreement taken seriously by either side? How did Ibn Saud's Wahhabi supporters feel about him essentially subjecting himself to an infidel empire?
Only on paper; the British never really had any agents or attempted to assert control over the al-Saud. The primary motive for the treaty was to get the Saudis to recognize British sovereignty over Kuwait (which the al-Saud had tried to conquer several times).
The British had no interest in Nejd (the area around Riyadh) and were basically happy to leave the Saudis alone as long as they didn't disturb British interests. The majority of ibn Saud's followers probably never really knew the agreement even existed; I certainly doubt the king himself felt any fealty to Britain!