The British government concluded “protection and friendship” treaties with nine tribes surrounding Aden, whereas they would remain independent from British interference in their affairs as long as they do not conclude treaties with foreigners (non-Arab colonial powers). Aden was declared a free zone in 1850. With emigrants from India, East Africa, and Southeast Asia, Aden grew into a world city. In 1850, only 980 Arabs were registered as original inhabitants of the city. The English presence in Aden put them at odds with the Ottomans. The Turks asserted to the British that they held sovereignty over the whole of Arabia, including Yemen as the successor of Mohammed and the Chief of the Universal Caliphate.
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.