They were four words that became the national rallying cry of a new civil rights movement: “Hands up, don’t shoot.” Protesters chanted it, arms raised, in cities across the country in solidarity for Michael Brown, the black teenager who some witnesses said was surrendering when he was shot and killed by a white police officer in Ferguson, Mo. The slogan was embraced by members of Congress, recording artists and football players with the St. Louis Rams. It inspired posters and songs, T-shirts and new advocacy groups, a powerful distillation of simmering anger over police violence and racial injustice in Ferguson...