After the 1948 election victory of the Afrikaner-dominated National Party with its apartheid policy of racial segregation, Mandela was prominent in the ANC's 1952 Defiance Campaign and the 1955 Congress of the People, whose adoption of the Freedom Charter provided the fundamental program of the anti-apartheid cause. During this time, Mandela and fellow lawyer Oliver Tambo operated the law firm of Mandela and Tambo, providing free or low-cost legal counsel to many blacks who would otherwise have been without legal representation.
Initially committed to non-violent mass struggle, Mandela was arrested with 150 others on 5 December 1956, and charged with treason. The marathon Treason Trial of 1956–61 followed, and all were acquitted. From 1952–59 the ANC experienced disruption as a new class of Black activists (Africanists) emerged in the townships demanding more drastic steps against the National Party regime. The ANC leadership of Albert Luthuli, Oliver Tambo and Walter Sisulu felt not only that events were moving too fast, but also that their leadership was challenged. They consequently bolstered their position by alliances with small White, Coloured and Indian political parties in an attempt to appear to have a wider appeal than the Africanists. The 1955 Freedom Charter Kliptown Conference was ridiculed by the Africanists for allowing the 100,000-strong ANC to be relegated to a single vote in a Congress alliance, in which four secretary-generals of the five participating parties were members of the secretly reconstituted South African Communist Party (SACP), strongly adhering to the Moscow line.
In 1959, the ANC lost its most militant support when most of the Africanists, with financial support from Ghana and significant political support from the Transvaal-based Basotho, broke away to form the Pan Africanist Congress (PAC) under Robert Sobukwe and Potlako Leballo. Following the massacre of PAC supporters at Sharpeville, in March 1960, and the subsequent banning of PAC and ANC, the ANC/SACP followed the African Resistance Movement (renegade liberals) and PAC into armed resistance. Luthuli, criticised for inertia, was peripheralised, and the ANC/SACP used the All-In African Conference of 1961, where all parties met to decide a joint strategy, for Mandela to issue a dramatic call to arms, announcing the formation of Umkhonto we Sizwe, modeled on the Jewish guerrilla movement, Irgun, and commanded by Mandela with SACP Jewish activists Denis Goldberg, Lionel "Rusty" Bernstein, and Harold Wolpe.
Mandela then left the country secretly and met African leaders in Algeria and elsewhere. Startled to discover the depth of support for the PAC and the widespread belief that the ANC was a small Xhosa tribal association manipulated by White communists, Mandela returned to South Africa determined to reassert the African nationalist element in the Congress Alliance. It is widely suspected that a heated discussion with the communist leaders over this issue led to his subsequent betrayal and arrest near Howick. Mandela glossed over these events in his autobiography but at least one prominent SACP activist associated with him at that time was cold-shouldered on his return to South Africa.