June 1963 he defected from the Army and joined the South Sudan resistance movement against the Government of Sudan. September 1963 he founded Anya nya the military wing of the resistance movement, named after a deadly poison. Anya-nya reinvigorated the movement that erupted on 18 August 1955 and continued the fight against the Sudanese government in the First Sudanese Civil War, which after the mutiny of 1955, really actively began in 1963. Among Joseph Lagu's junior officers in Anya-nya was John Garang who was recruited in October 1970 and was later to become the chief architect of the second civil war.
Peace process
The war ended in 1972, after a peace agreement was signed in Addis Ababa by the Sudanese government led by President Gaafar Nimeiry and the South Sudan Liberation Movement, political wing of the resistance founded by Joseph Lagu when he took overall control of the entire southern resistance in January 1971. The agreement was signed under the auspices of emperor Haile Selassie. The Addis Ababa Agreement granted regional autonomy to Southern Sudan and ensured that Anya-nya and its political arms would be absorbed into the national army, police force and the newly formed national government. Joseph Lagu, who rejoined the Sudanese armed forces with the rank of Major General, stayed on in the Army to ensure a smooth merger of the disparate forces. The ten years following the agreement gave the country the longest period of relative peace in Sudan's turbulent history, plagued as it was by coup d'etats and civil wars. It also gave Southern Sudan a chance at developing democratic institutions in its own autonomous context.
Public life
After leaving military service he entered public life. He was elected by popular vote to the Presidency of the High Executive Council of the Southern Sudan Autonomous Region in 1978 and in 1982 was appointed 2nd Vice President of the Republic. In 1985 when Nimeiry's government was toppled by his defence minister, Gen. Mohammed Ahmed Suar-El-Dahab, Joseph Lagu also lost his position as 2nd Vice President. Relocating to the UK with his family, Joseph Lagu kept quite a low political profile, though he sometimes appeared on political rallies. The brief military rule that succeeded the coup was followed by a democratically elected government under the Premiership of Sadiq al-Mahdi. Joseph Lagu was appointed Roving Ambassador by the elected Prime Minister Sadiq al-Mahdi. Al Mahdi's government was in turn overthrown by Gen. Omar al-Bashir who has managed to maintain his hold on power since the coup in 1989. Joseph Lagu's post as Roving Ambassador continued under the new regime having been confirmed by President al-Bashir himself. Joseph Lagu was appointed as Sudanese Ambassador to the UN between September 1990 and January 1992. He was then reappointed Roving Ambassador. He resigned from the position in May 1998 after he was appointment as Presidential Advisor, a post that he declined requesting he be relieved of all official duty so as to become a private citizen and an independent voice. In 2006 he completed writing his memoirs—Sudan Odyssey Through a State from Ruin to Hope.