'''Imam Yahya hamid ed-Din al-Mutawakkil''' was ruling the northern highlands independently from 1911. After the Ottoman departure in 1918, he sought to recapture the lands of his Qasimid ancestors. He dreamed of Greater Yemen stretching from Asir to Dhofar. These schemes brought him into conflict with the ''de facto'' rulers in the territories claimed, namely the Idrisids, Ibn Saud, and the British government in Aden. The Zaydi imam did not recognize the Anglo-Ottoman border agreement of 1905 on the grounds that it was made between two foreign powers occupying Yemen. The border treaty effectively divided Yemen into north and south. In 1915, the British signed a treaty with the Idrisids guaranteeing their security and independence if they would fight against the Turks. In 1919, Imam Yahya hamid ed-Din moved southward to "liberate" the nine British protectorates. The British responded by moving quickly towards Tihama and occupying al-Hudaydah. Then they handed it over to their Idrisi allies. Imam Yahya attacked the southern protectorates again in 1922. The British bombed Yahya's tribal forces using aircraft to which the tribes had no effective counter.