Sudan is geographically the largest country on the African continent and is composed principally of Arabs in the north and black Africans in the south, creating a population of mixed ancestry. Present-day Sudan was part of the region known as Nubia, which was the seat of a civilization akin to the Pharonic civilization of Egypt. The Christian kingdom of Napta gave way to the Arab/Muslim invasion, which gave rise to local kingdoms in Seunar and Tegali. These disintegrated before falling under Egyptian rule, Turkish-Egyptian rule, and British-Egyptian sovereignty. While the history and culture of Sudan is deeply intertwined with that of Egypt, northern Sudan retains much Islamic culture, while the south retains more traditional customs and has a substantial Christian minority. Following independence, the country experienced a transitional period of Sudanization – the replacement of foreign government and military officials with citizens – that further emphasized the geographic and social differences between the north and south. The gap between the north and south continued to grow despite the discovery of petroleum in the late 1970s and early 1980s. Drought, lack of resources, and decades of civil war led to widespread famine and environmental issues. Subsequently, large numbers of refugees crossed into neighboring countries as a result of this destruction and the offensive led against the Sudanese People’s Liberation Army (SPLA) in 1994. Negotiations between the government in Khartoum and the SPLA are proceeding. The identity of the Sudanese state with Islam is also the subject of disputes among Sudanese Muslims.