Initially opposed to working with other antiapartheid groups, during the 1952 Defiance Campaign Mandela changed his mind and thereafter advocated united action across South Africa's racial divides to challenge apartheid policies. In December 1952 he was convicted under the 1950 Suppression of Communism Act and received a nine-month suspended sentence. Despite government-imposed restrictions limiting his movement and banning him from political meetings, he continued to work as an ANC leader and was responsible for the development of a contingency plan under which the ANC would continue to work clandestinely in the event of a state crackdown. In December 1956 he was among 156 activists charged with treason against the state. The subsequent Treason Trial, in which Mandela and the others were charged with plotting a revolution, lasted until 1961. Mandela was acquitted.
After the March 1960 Sharpeville massacre in which sixty-seven blacks were killed during an antiapartheid demonstration, the government banned the ANC and other dissident groups. A year later, to evade arrest, Mandela went underground. In June 1961 the ANC launched an armed struggle against the state, with Mandela now leading the Umkhonto we Sizwe (Spear of the Nation), the ANC's armed wing. Mandela coordinated a sabotage campaign and planned for a guerrilla insurgency. In January 1962 he secretly left the country and traveled to Ethiopia, Algeria, and London to solicit support and receive guerrilla training. When he returned to South Africa that summer, he was captured on 5 August.
In the case that followed, Mandela was convicted on charges of incitement and illegally leaving the country and was sentenced to five years in prison. While he was in prison, police raided the ANC's underground headquarters, and many ANC leaders were arrested. This development brought Mandela back to court in the Rivonia Treason Trial, in which he was sentenced to life imprisonment on 12 June 1964. Mandela spent eighteen years at Robben Island Prison, off Cape Town, before being transferred to Pollsmoor Prison in 1982.
Amid growing external and internal pressures, South Africa's apartheid regime began to unravel in the late 1980s. Finally, on 18 February 1990, Mandela was released by President F. W. de Klerk, just days after de Klerk had lifted the ban on the ANC and other antiapartheid groups. Mandela succeeded the ailing Tambo as president of the ANC in 1991, the same year in which the government repealed the last of the apartheid laws. For their efforts to end apartheid, both Mandela and de Klerk shared the 1993 Nobel Peace Prize.
In May 1994, following the country's first national elections in which all races could vote, Mandela became the first black president of South Africa, a post he held until 1999.
Mandela, Nelson. Long Walk to Freedom: The Autobiography of Nelson Mandela. Randburg, South Africa: Macdonald Purnell, 1994.; Sampson, Anthony. Mandela: The Authorized Biography. New York: Knopf, 1999.; Suttner, Raymond. "The African National Congress (ANC) Underground: From the M-Plan to Rivonia." South African Historical Journal 49 (2003): 123–146.