The SPLA was founded as a guerrilla movement against the government of Sudan in 1983 and was a key participant of the Second Sudanese Civil War, led by John Garang.

As of 2018[update], the army was estimated to have 185,000 soldiers as well as an unknown number of personnel in the small South Sudan Air Force.

[10] In the mid-1980s the SPLA armed struggle blocked development projects of the Sudanese government, such as the Bentiu Oil Fields.

[24] In September 1989, the ruling Revolutionary Command Council for National Salvation (RCC) invited different sectors to a 'National Dialogue Conference', but the SPLA refused to attend.

It captured various towns, such as Bor, Waat, Maridi, Mundri, Yambio, Kaya, Kajo Keji, Nimule, Kapoeta, Torit, Akobo and Nasir.

The Ethiopian government had provided the SPLA with military supplies, training facilities and a safe haven for bases for 18 years.

[18] A split in the SPLA had simmered since late 1990, as Lam Akol and Riek Machar began to question Garang's leadership.

[35] In 2004, a year before the Comprehensive Peace Agreement (CPA), the Coalition to Stop Child Soldiers, estimated that there were between 2,500 and 5,000 children serving in the SPLA.

In 2005, Garang restructured the top leadership of the SPLA, with a Chief of General Staff, Lt. Gen. Oyay Deng Ajak, and four Deputy Chiefs of General Staff: Maj. Gen. Salva Mathok Gengdit (Administration), Maj. Gen. Bior Ajang Aswad (Operations), Maj. Gen. James Hoth Mai (Logistics) and Maj. Gen. Obuto Mamur Mete (Political and Moral Orientation).

[38] The four independent brigades grouped SPLA forces in Bor (Khoriom, 104, and 105 Battalions mainly), Southern Blue Nile, the Nuba Mountains (South Kordofan) and Raja (Western Bahr el Ghazal).

Probably more important than the reorganisation was the Juba Declaration, signed by Salva Kiir and General Paulino Matiep on 8 January 2006.

Matiep commanded the South Sudan Defence Forces (SSDF), the largest and best-equipped militia (about 50,000 men) that remained beyond the SPLA's control.

At about the same time, the legislature voted to double infantrymen's base pay from the equivalent of $75 a month (the rate under Khartoum's control) to $150.

[citation needed] The unification of the two largest armed groups in the region seriously weakened Khartoum’s control of southern Sudan.

Dim died in a plane crash in 2008 alongside his wife, Josephine Apieu Jenaro Aken, and other SPLA officers.

[44] Officially, this move did take place, in 2008, with the 10th Division relocating its headquarters to Guffa, five kilometers south of the Blue Nile-Upper Nile border, and most of its troops to al-Fuj, Yafta and Marinja on the southern side.

[46] After the fighting began, former SPLA 9th and 10th Division fighters proclaimed themselves the Sudan People's Liberation Movement-North (SPLA-N), under Malik Agar as Chairman and Commander-in-Chief.

This expansion coincided with the completion of the General Headquarters at Bilpam, built by DynCorp with funds from the U.S. State Department’s Africa Peacekeeping Program (AFRICAP).

[49] On 15 December, 2013, fighting broke out in Juba between different factions of the armed forces in what the South Sudanese government described as a coup d'état.

[51] The ferocity with which people were chased into the swamps to be killed was aimed at annihilating the SPLM/A-in-Opposition's support, and led to systematic destruction of villages and towns.

The Panel of Experts wrote in 2016, "While other tribes are represented in SPLA, they are increasingly marginalized, rendering the multi-tribal structure of the army largely a façade that obscures the central role that Dinka now play in virtually all major theatres of the conflict".

(S/2016/963, 8) On May 16, 2017, Kiir announced a restructure of the army and change of name to the South Sudan Defence Forces (SSDF).

[55] According to Professor Joel Isabirye, the change of name would shift the discourse from the era of liberation, which had now concluded, to one of national defence, which is ongoing – with the focus on defending the country against external aggression.

[56] The negotiations stalled over disagreement among the parties about power sharing, future security arrangements and whether Riak Machar could return from exile to political life in South Sudan.

In early May 2018, a two-day meeting of the Parties to the Revitalized Agreement on the Resolution of the Conflict in South Sudan (R-ARCSS) started in Addis Ababa.

[57] The renaming occurred ten days before implementation of new security arrangements, which include the reunification of the national army.

[58] As of 2018, the army was estimated to have 185,000 soldiers as well as an unknown number of personnel in the small South Sudan Air Force.

[59] According to the CIA World Factbook as of June 2020[update], "under the September 2018 peace agreement, all armed groups in South Sudan were to assemble at designated sites where fighters could be either disarmed and demobilized, or integrated into unified military and police forces; the unified forces were then to be retrained and deployed prior to the formation of a national unity government; all fighters were ordered to these sites in July 2019, but as of April 2020 this process had not been completed".

[61] UNMISS has been in the country since 2011, aiming to consolidate peace and achieve security to allow economic growth and political stability.

After the restructure as SSPDF, Malong was superseded by James Ajongo Mawut (May 2017–April 2018), with the position now referred to as "chief of defence force(s)".