rail lines to link key points in northern Sudan but services did not reach more remote areas. Port Sudan opened in 1906, replacing Sawakin as the country's principal outlet to the sea. In 1911 the Sudanese government and the private Sudan Plantations Syndicate launched the Gezira Scheme (Gezira is also seen as Jazirah) to provide a source of high-quality cotton for Britain's textile industry. An irrigation dam near Sennar, completed in 1925, brought a much larger area in Al Jazirah under cultivation. Planters sent cotton by rail from Sannar to Port Sudan for shipment abroad. The Gezira Scheme made cotton the mainstay of the country's economy and turned the region into Sudan's most densely populated area. Egyptian independence, fate of Sudan In 1922, Britain renounced the protectorate and approved Egypt's declaration of independence. However, the 1923 Egyptian constitution made no claim to Egyptian sovereignty over Sudan. Subsequent negotiations in London between the British and the new Egyptian government foundered on the Sudan question. Inflamed by the failure of the talks, nationalists rioted in Egypt and in Sudan where a minority supported union with Egypt. In November 1924, Sir Lee Stack, governor general of Sudan and Sirdar, was assassinated in Cairo. Britain ordered all Egyptian troops, civil servants, and public employees withdrawn from Sudan. In 1925, Khartoum formed the 4,500-man Sudan Defence Force (SDF) under Sudanese officers to replace Egyptian units. Indirect rule Sudan was relatively quiet in the late 1920s and 1930s. During this period, the colonial government favored indirect rule, which allowed the British to govern through indigenous leaders. In Sudan, the traditional leaders were the shaykhs (of villages, tribes, and districts) in the north and tribal chiefs in the south. The British first delegated judicial powers to shaykhs to enable them to settle local disputes; then, gradually, they allowed the shaykhs to administer local government under the supervision of British district commissioners. The number of Sudanese recognizing these leaders and the degree of authority they wielded varied considerably. The mainstream of political development, represented by other local leaders and Khartoum's educated elite, disapproved of indirect rule. In their view, it prevented the country's unification, exacerbated tribalism in the north, and in the south served to buttress a less-advanced society against Arab influence. Indirect rule also implied government decentralization, which alarmed the educated elite who had careers in the central administration and envisioned an eventual transfer of power from