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.
In 1925, Imam Yahya captured al-Hudaydah from the Idrisids. He continued to follow and attack the Idrisids until Asir fell under the control of the imam’s forces, forcing the Idrisi to request an agreement that would enable them to administer the region in the name of the imam. Imam Yahya refused the offer on the grounds that the Idrisis were of Moroccan descent. According to Imam Yahya, the Idrisis, along with the British, were nothing but recent intruders and should be driven out of Yemen permanently. In 1927, Imam Yahya’s forces were about 50 km (30 mi) away from Aden, Taiz, and Ibb, and were bombed by the British for five days; the imam had to pull back. Small Bedouin forces, mainly from the Madh’hij confederation of Marib, attacked Shabwah but were bombed by the British and had to retreat.
The Italian Empire was the first to recognize Imam Yahya as the King of Yemen in 1926. This created a great deal of anxiety for the British, who interpreted it as recognition of Imam Yahya’s claim to sovereignty over Greater Yemen, which included the Aden protectorate and Asir. The Idrisis turned to Ibn Saud seeking his protection from Yahya Muhammad Hamid ed-Din. However, in 1932, the Idrisis broke their accord with Ibn Saud and went back to Imam Yahya seeking help against Ibn Saud himself, who had begun liquidating their authority and expressed his desire to annex those territories into his own Saudi domain. Imam Yahya demanded the return of all Idrisi dominion. That same year, a group of Hejazi liberals fled to Yemen and plotted to expel Ibn Saud from the former Hashemite Kingdom of Hejaz, which had been conquered by the Saudis seven years earlier. Ibn Saud appealed to Britain for aid. The British government sent arms and airplanes . The British were anxious that Ibn Saud’s financial difficulties may encourage the Italian Empire to bail him out. Ibn Saud suppressed the Asiri rebellion in 1933, after which the Idrisids fled to Sana’a. Negotiations between the Imam Yahya Hamid ed-Din and Ibn Saud proved fruitless. After the 1934 Saudi-Yemeni war, Ibn Saud announced a ceasefire in May 1934. Imam Yahya agreed to release Saudi hostages and the surrender of the Idrisis to Saudi custody. Imam Yahya ceded the three provinces of Najran, Asir, and Jazan for 20 years and signed another treaty with the British government in 1934. The imam recognized the British sovereignty over Aden protectorate for 40 years. Out of fear for Hudaydah, Yahya did submit to these demands. According to Bernard Reich, Professor of Political Science and International Affairs at George Washington University, Yahya could have done better by reorganizing the Zaydi tribes of the northern highlands as his ancestors did against the Turks and British intruders and turn the lands they captured into another graveyard.
Starting in 1890, hundreds of Yemeni people from Hajz, Al-Baetha, and Taiz migrated to Aden to work at ports, and as laborers. This helped the population of Aden once again become predominantly Arab after, having been declared a free zone, it had become mostly foreigners. During World War II, Aden had increasing economic growth and became the second-busiest port in the world after New York City. After the rise of labor unions, a rift was apparent between the sectors of workers and the first signs of resistance to the occupation started in 1943. Muhammad Ali Luqman founded the first Arabic club and school in Aden, and was the first to start working towards a union.