Martin Luther King Jr. looks through the bars of a Birmingham, Alabama, cell in April 1963. Civil rights leaders had been campaigning for desegregation in Birmingham. King was jailed for holding marches without a permit. While in jail, he responded to a published letter from moderate white preachers criticizing the campaign with his famous “Letter From a Birmingham Jail.” "One has a moral responsibility to disobey unjust laws," King said in the letter, which was written on scraps of paper and smuggled out in installments. (National Archives)