History of Sudan
The modern Republic of Sudan was formed in 1956 and inherited its boundaries from Anglo-Egyptian Sudan, established in 1899. For times predating 1899, usage of the term "Sudan" for the territory of the Republic of Sudan was somewhat anachronistic, and may have referred to the more diffuse concept of the Sudan.
History of South Sudan
In 1983, President of Sudan Gaafar Nimeiry declared all Sudan an Islamic state under Shari'a law, including the non-Islamic majority southern region. The Southern Sudan Autonomous Region was abolished on 5 June 1983, ending the Addis Ababa Agreement. In direct response to this, the Sudan People's Liberation Army/Movement (SPLA/M) was formed under the leadership of John Garang, and the Second Sudanese Civil War erupted. Several factions split from the SPLA often along ethnic lines and were funded and armed by Khartoum, with the most notable being the SPLA-Nasir in 1991 led by Riek Machar.