Why did they eventually let Nelson Mandela get out of jail?

Nelson Mandela was arrested 5 August 1962 and released from prison 2 February 1990. Pro-apartheid South African president, P.W. Botha, had a stroke in 1989 and Botha's replacement, Frederik Willem de Klerk, freed Mandela. Botha had once offered Nelson Mandela release from jail in 1985, but only on the condition that Mandela would stop the armed conflict. Nelson Mandela chose to stay in prison as he believed it was the right thing to do in order to put an end to apartheid.