During World War I Ibn Sa'ud entered into a treaty with the British (December 1915), accepting protectorate status and agreeing to make war against Ibn Rashid, who was being supported by the Turks. But despite British arms and a subsidy of £5,000 a month from the British government (which continued until 1924) he was inactive until 1920, arguing that his subsidy was insufficient. During 1920-22, however, he marched against Ibn Rashid and extinguished Rashidi rule, doubling his own territory but without significantly increasing his meagre revenue.
The Ikhwan were also instrumental in securing the provinces ... Ha'il, in the north of the peninsula, along the borders of Transjordan and Iraq (1921).
Abd al Aziz restrained the Ikhwan and managed to direct them toward Hail, which they took easily in 1921. The Ikhwan went beyond Hail, however, and pushed into central Transjordan where they challenged Hussein's son, Abd Allah, whose rule the British were trying to establish after the war. At this point, Abd al Aziz again had to rein in his troops to avoid further problems with the British...
He sent the Ikhwan against the Al Rashid stronghold at Hail, which was captured with little difficulty in 1921.
In 1921 Ibn Sa'ud defeated the forces of Muhammad ibn Talal, the last Rashidi emir, and annexed the whole of northern Arabia, occupying Al-Jawf and Wadi As-Sirhan in the following year.