In the late 1960s, Kiir joined the Anyanya battalion in the First Sudanese Civil War. By the time of the 1972 Addis Ababa Agreement, he was a low-ranking officer. In 1983, when Dr John Garang joined an army mutiny he had been sent to put down, Kiir and other Southern leaders joined the rebel Sudan People's Liberation Movement (SPLM) in the second civil war. Dr. Garang De Mabior had advanced military knowledge and experience from both the United States and the Sudan, and Kiir served as his deputy. In 1997, Kiir commanded the SPLA troops that took part in Operation Thunderbolt, a very successful rebel offensive during which most of Western Equatoria was captured by the SPLA. Kiir eventually rose to head the SPLA, the SPLM's military wing when Dr. John Garang was killed in a helicopter crash. Rumours to remove Kiir from his post as SPLA Chief of Staff in 2004 nearly caused the organization to split.