Nelson Mandela was a founder of the armed wing of the African National Congress, called Umkhonto we Sizwe (Spear of the Nation, often abbreviated as MK). This group was founded in response to the Sharpeville Massacre, where 69 unarmed protesters were shot dead in response for demanding a change to the pass laws (a legal document which restricted black's right to live and work where they chose). Under his leadership before his imprisonment the group was responsible for a string of bombings on military and infastructure targets.

Now Nelson Mandela himself was on trial twice. The first time was in regards to charges of treason, during the Treason Trail of 1956. The political climate in South Africa was tense as the Nationalist party was cementing the foundation of Apartheid and a formerly de-facto system of segregation and discrimination was becoming de jure. The ANC and Mandela had organised various marches and demonstrations against this. Much like America at the same time South Africa also underwent it's own ''red scare'', the ''rooi gevaar'' which was further mixed and complicated by the threat of the native bantu population, the so called ''swart gevaar''. South Africa's nationalist government were far-right white supremacist and terrified of the threat of a communist and/or black uprising. The ANC's response was peaceful protest and civil disobedience, much in the vein of the civil rights movement in the United States. So a load of black nationalists and white sympathisers were rounded up and put on trial. They were a formidable group, who had some of the best South African legal minds on their case. The government more or less embarrassed itself and failed to gain any convictions. Nelson Mandela was acquitted during the Treason trail and walked a free man.

Following his release Nelson Mandela went underground. The ANC went into a period of transformation as it became apparent that civil disobedience and strikes weren't working. In the wake of Sharpeville, Nelson Mandela advocated an armed campaign, but the idea was shot down by more senior ANC leaders. After a long while of negotiation the MK was formed, but officially as a separate organisation while the ANC itself maintained a non-violent approach.

Nelson Mandela had reservations over the use of violence, so in his words with regards to MK's strategy at first...

it made sense to start with the form of violence that inflicted the least harm against individuals: sabotage. Because it did not involve loss of life it offered the best hope of reconciliation among the races afterwards''.

Only once this method has failed would violence be escalated. MK first struck on the Day of the Vow, 16th December, which was a national holiday to mark a victory of the Afrikaners over the Zulus at Blood river. It targeted an electricity substation and government offices. Between then and his capture they carried out a number of other raids, mainly aimed at infrastructure. He also underwent limited military training in Ethiopia.

Nelson Mandela was captured in August 1962. He was charged with illegally leaving the country and incitement. Later would follow the Rivonia trail in regards to MK activities, primarily sabotage. He gave a speech from the dock justifying the decision to resort to violence, and denying being terrorists as they fought for majority rule and were non-racialist. He was found guilty and convicted.

During the 1970s and 1980s the struggle against Apartheid became ugly. MK was responsible for various attacks on civilians, and many bombings of civilian targets. Within the townships the MK fought a horrible war against other black factions such as the Inkatha Freedom Party. I don't wish to devolve into the subject matter, but search the term ''necklacing'' for further information (NSFL). Many people say that Nelson Mandela was responsible for this either due to his forming of the MK and starting the armed struggle, or as a ''shadow leader'' from behind bars. There is little evidence for the later allegation, most evidence points towards MK being an independent organisation run from members in exile opposed to from Robben Island or Pollsmoor Prison. Following Mandela's release in 1990 the activities of the MK wound down, and the ANC, at least officially, went back towards non-violent methods in negotiations with the government in regards to an end of Apartheid and majority rule. These were still turbulent times with a lot of violence happening between different groups and factions. The Shell House Massacre of 1994 happened in response to supporters of the IFP surrounding the ANC headquarters. Conflicting reports state that ANC bodyguards fired on the group in response to a threat or fear of the group rushing the building, or due to panic and confusion. However Nelson Mandela admitted that he authorised his bodyguards to use force to protect the building. At least 19 people died.

Ultimately Nelson Mandela committed acts of violence. However most would argue that they were proportional and justified in the context of the struggle against Apartheid. And once the government released him and started negotiations ''officially'' put an end to the armed campaign. Afterwards he practised and preached peace and reconciliation in the new South Africa. Horrible atrocities were committed by all sides during the Apartheid era, but personally I think that Nelson Mandela comes off relatively clean.

Sources: Long Walk to Freedom - Nelson Mandela

South Africa: First Man, Last Nation - R.W. Johnson.

Coming to Terms: South Africa's Search for Truth - Martin Meredith

(edit: I'm terrible at formatting, few typos and expanding on other issues)