The tribal chiefs were difficult to appease and an endless cycle of violence curbed Ottoman efforts to pacify the land. Ahmed Izzet Pasha proposed that the Ottoman army evacuate the highlands and confine itself to Tihama, and not unnecessarily burden itself with continuing military operation against the Zaydi tribes. The hit-and-run tactics of the northern highlands tribesmen wore out the Ottoman military. They resented the Turkish ''Tanzimat'' and defied all attempts to impose a central government upon them. The northern tribes united under the leadership of the House of Hamidaddin in 1890. Imam Yahya Hamidaddin led a rebellion against the Turks in 1904; the rebels disrupted the Ottoman ability to govern. The revolts between 1904 and 1911 were especially damaging to the Ottomans, costing them as many as 10,000 soldiers and as much as 500,000 pounds per year. The Ottomans signed a treaty with imam Yahya Hamidaddin in 1911. Under the treaty, Imam Yahya was recognized as an autonomous leader of the Zaydi northern highlands. The Ottomans continued to rule Shafi'i areas in the mid-south until their departure in 1918.