The Southern freedom movement made imprisonment itself a political instrument. From Rosa Parks's arrest in December 1955 through the Montgomery bus boycott — whose leaders, including Martin Luther King Jr., were indicted en masse under an old anti-boycott statute — activists accepted jail as the price and the proof of their witness. King would be arrested some thirty times; his April 1963 confinement in Birmingham, for marching in defiance of a state-court injunction, produced the "Letter from Birmingham Jail," the era's most enduring defense of civil disobedience against unjust law.
Birmingham also showed the system's capacity for mass incarceration of the movement: during the May 1963 Children's Crusade, thousands of schoolchildren were arrested and held in improvised pens, filling the jails exactly as the campaign intended. The convictions of that spring followed the movement for years — the Supreme Court upheld King's contempt sentence in Walker v. City of Birmingham (1967), sending him back to jail in Birmingham in the last year of his life.