Watching the Sunday talk shows I heard a conservative (possibly Mary Matlin) refer to Mandela's "terrorist past" as one of the reasons why conservatives were so wary about him in the 1980s. Nelson Mandela was the founder and leader of the African National Congress's (ANC) armed wing, Umkhonto weSizwe (Spear of the Nation) or MK for short. MK began its sabotage campaign in December 1961 and it lasted for about 18 months before the South African Police managed to discover the headquarters of the organization at a farm in the Rivonia suburb of Johannesburg. During the sabotage campaign the MK attacked mainly symbolic targets such as electrical pylons, postboxes, and other infrastructure. Pro-German Afrikaner organizations carried out a similar campaign during World War II. At the Rivonia treason trial in 1964 Mandela and his comrades were sentenced to life in prison (except for a couple one of whom was white who received ten-year sentences). Mandela had actually been in prison since the second half of 1962 when, having been betrayed by the CIA, he was arrested for going abroad without a passport--which the government would not have issued him--and sentenced to five years.
So in the period in which Mandela actually had control over the organization it only engaged in sabotage. I like to divide organized political violence, or armed struggle as it is euphemistically known, into three different categories: sabotage, guerrilla warfare, and terrorism. Sabotage involves attacks on property but not attacks on people. Guerrilla warfare involves attacks on "legitimate targets," which are normally considered to be police and military personnel--those directly responsible for guarding the regime in power. Terrorism involves attacks on random civilians. Now there are grey areas: assassinations of government officials can be seen as guerrilla warfare rather than terrorism, but some consider this to be terrorism. Mao Tse-Tung listed terrorism as the second stage of revolution followed by guerrilla warfare--so obviously the two types of warfare are not incompatible. Historian Walter Lacqueur defined any movement that was not attempting to replicate itself on a large scale and control territory as terrorist. By this definition MK was a terrorist organization. But he is in the minority, most academics define terrorism in term of targeting and legal norms: targeting of ordinary civilians is terrorism, targeting of military or economic targets is not.
Later after the Soweto Rebellion of late 1976 the liberation movements abroad got a new influx of young South African volunteers eager for revenge against the regime. They were specifically trained that ordinary whites were not legitimate targets and that the goal of the armed struggle was to eliminate white supremacy without replacing it with African or black supremacy. In the late 1970s the armed struggle amounted to a few guerrilla attacks on police stations, but nothing that was really a threat to the regime. In June 1980 MK managed to blow up one of South Africa's Sasol coal-to-oil conversion plants, which used technology developed by Germany during WWII to help evade sanctions. For the next decade MK carried out an armed struggle that did have some spectacular moments such as the bombing of Air Force headquarters in Pretoria and an attack on a nuclear plant. But they still were not a real threat to the regime.
The real terrorism in South Africa was not carried out by MK. It was carried out by state agents involved in a campaign of assassination against anti-apartheid figures, both black and white, inside South Africa and in the surrounding countries and even in Paris. The details confirming that these acts were carried out by state agents were revealed under oath before the Truth and Reconciliation Commission from 1997 to 2002. During the 1980s there was also terrorism carried out by black youths in the townships who murdered suspected informers and some local black officials, usually by necklacing--the setting on fire of gasoline-soaked tires hanging around the necks of the victims. This was not encourage by ANC officials, at least not publicly, but some connected with the United Democratic Front, the legal internal front for the ANC, including Winnie Mandela did encourage these killings. Winnie Mandela was arguably a terrorist; her husband was never a terrorist.
But there is another interesting question that conservatives worried about. From the 1940s onwards the ANC had a strategic partnership with the South African Communist Party. Many Communists held important positions in the ANC, some openly and some covertly. No biography of Mandela has ever acknowledged his membership in the SACP. But noted South African scholar R.W. Johnson here claims that Mandela had always been a Communist. If this is true, it gives some credibility to conservative fears about the ANC in Britain and the United States in the 1980s. But it should be remembered that it was Thabo Mbeki, the Moscow-trained economist who was the son of SACP leader Govan Mbeki, who as president after Mandela and vice-president under Mandela instituted the policy of forming a partnership with business.