A crowd of onlookers gawked from the sidewalk as four young black men dressed in black leather jackets and berets leaped from a Volkswagen, each of them wielding shotguns with bandoliers strapped across their bodies. The young men surrounded two white police officers who had accosted a black man and had him spread-eagled against a building. The young men did not say a word as the police officers watched them nervously, their eyes fixed on the shotguns....