Why did Mandela and others make the decision to form the organization Umkhonto you Sizwe?

"Firstly, we believed that as a result of Government policy, violence by the African people had become inevitable, and that unless responsible leadership was given to canalize and control the feelings of our people, there would be outbreaks of terrorism which would produce an intensity of bitterness and hostility between the various races of this country which is not produced even by war. Secondly, we felt that without violence there would be no way open to the African people to succeed in their struggle against the principle of white supremacy."(Mandela 2) This taken from his book I'm Prepared to Die