The Islamization of the Sudan region (Sahel) encompasses a prolonged period of religious conversion, through military conquest and trade relations, spanning the 8th to 16th centuries. Following the 7th century Muslim conquest of Egypt and the 8th-century Muslim conquest of North Africa, Arab Muslims began leading trade expeditions into Sub-Saharan Africa, first towards Nubia, and later across the Sahara into West Africa. Much of this contact was motivated by interest in trans-Saharan trade, particularly the slave trade. The proliferation of Islamic influence was largely a gradual process. The Christian kingdoms of Nubia were the first to experience Arab incursion starting in the 7th century. They held out through the Middle Ages until the Kingdom of Makuria and Old Dongola both collapsed in the early 14th century. Sufi orders played a significant role in the spread of Islam from the 9th to 14th centuries, and they proselytized across trade routes between North Africa and the sub-Saharan kingdoms of Ghana and Mali. They were also responsible for setting up zawiyas on the shores of the River Niger. The Mali Empire underwent a period of internally motivated conversion following the 1324 pilgrimage of Musa I of Mali. Subsequently Timbuktu became one of the most important Islamic cultural centers south of the Sahara. Alodia, the last holdout of Christian Nubia, was destroyed by the Funj in 1504. During the 19th century the Sanusi order was highly involved in missionary work with their missions focused on the spread of both Islam and textual literacy as far south as Lake Chad. Consequently, much of contemporary Sudan region is Muslim. This includes the Republic of Sudan (after the secession of Christian-majority South Sudan), the northern parts of Chad and Niger, most of Mali, Mauritania and Senegal. The problem of slavery in contemporary Africa remains especially pronounced in these countries, with severe divides between the Arabized population of the north and dark-skinned Africans in the south motivating much of the conflict, as these nations sustain the centuries-old pattern of hereditary servitude that arose following early Muslim conquests. Ethnic strife between Arabized and non-Arab black populations has led to various internal conflicts in Sudan, most notably the War in Darfur, the Northern Mali conflict, and the Islamist insurgency in Northern Nigeria. The Arabs Contacts between Nubians and Arabs long predated the coming of Islam, but the Arabization of the Nile Valley was a gradual process that occurred over a