Nasir sits on the north bank of the Sobat River, about 30 kilometers from the Ethiopian border, in Nasir County of Upper Nile State. The Sobat is the great eastern tributary of the White Nile, draining much of Ethiopia's western highlands, and the town grew up where the river was crossable and the land around it supported cattle. Charles W. Gwynn, the British officer surveying the Ethiopia-Sudan frontier in March 1900, passed through Nasir and found 'a young Egyptian officer in charge of a small government post,' cut off from communications and anxiously awaiting the seasonal rise of the river to bring a supply steamer. The airstrip that exists today was reputedly built by the RAF in the 1930s as a fuel stop en route from Khartoum to Nairobi for British air operations across northeast Africa, a time when long-distance flight across the continent still depended on scattered earthen strips like this one.

On August 28, 1991, three senior SPLA commanders (Riek Machar, Lam Akol, and Gordon Kong Chuol) publicly broke with John Garang and announced the formation of SPLA-Nasir, named for the town where their headquarters sat. The split was partly about military strategy, partly about ethnic politics, and partly about Garang's autocratic leadership style. Its consequences were catastrophic. Within months, Nuer fighters aligned with Machar attacked Dinka civilians in the Bor Massacre of November 1991, killing around 2,000 people. The intra-SPLA conflict sharpened ethnic lines that had been blurred during earlier years of the war and shaped the fractures that would still be splitting the country thirty years later. Nasir also became a refugee reception center that year: in May 1991, when the Mengistu government fell in Ethiopia, Sudanese refugees who had been camped across the border fled home, and Nasir's population swelled from a few hundred to tens of thousands almost overnight.

UN Operation Lifeline Sudan used Nasir as a major distribution hub. WFP food shipments, UNICEF rinderpest vaccinations for cattle, meningitis vaccines for people, emergency feeding programs, seed and tool distributions. The town was part of the logistical backbone that kept civilian populations alive in areas the Sudanese government could not effectively supply. On February 12, 1998, a Sudanese Air Force Antonov An-32 crashed at Nasir Airport, killing senior officials including Sudan's Vice President Zubair Mohamed Salih. He was the senior-most figure to die in a Sudanese air accident during the war, and his death reshaped the political map in Khartoum for a time.

Since independence in 2011, Nasir has remained a flashpoint. In the aftermath of the South Sudanese Civil War that began in 2013, the town was the site of repeated clashes between the South Sudan People's Defence Forces and the Nuer White Army. The 2025 Nasir clashes marked another escalation, bringing the Vice President's home region and the shaky revitalized peace agreement back under strain. Through all of this, the airstrip the RAF built in the 1930s is still there. The Sobat still rises in June and falls in February. Cattle still move across the border to and from Ethiopia along tracks older than any of the wars. The people of Nasir, largely Nuer, have absorbed refugees, child soldiers, aid deliveries, armored vehicles, foreign aid workers, and various political projects that treated their town as a pin on a strategic map. What they have not done is agree to be only a pin. They have families, markets, mosques, churches, radios, and a stubborn sense that the place is theirs rather than a stage for the next confrontation.