RIOTS are rarely so widely anticipated. By 8pm on November 24th, when the prosecutor in Ferguson, Missouri, announced the grand jury’s decision not to charge a police officer with a crime for shooting an unarmed black teenager, Michael Brown, cops in riot gear were already in place and barriers surrounded municipal buildings. Mr Brown’s parents and Barack Obama called for calm. Yet soon America’s TV screens were full of burning police cars, crowds coughing on tear gas, and young black men throwing bricks and smashing shops. America’s history of racial injustice looked as potent as ever.