MLK Jr. was arrested in 1963 for protesting segregation in Alabama; and in his "Letter from Birmingham City Jail" was a very powerful, persuasive and effective document in response to an open letter in a local newspaper by white clergymen who criticized him.
In his letter, MLK explained his nonviolent protest actions as the duty of a responsible citizen.
The purpose of King's letter is to cause peace between all races in this nation to fight racism.