Chicago’s run of peaceful rallies outside the Democratic National Convention came to an end Tuesday night when a group of protesters marched into a line of police, setting off a violent scrum on Madison Street outside one of the city’s major commuter rail stations. The protesters walked directly into a large group of police officials, four officers deep, around 7:30 p.m. The officers began to push them the other way and yelled, “Move back, move back.” Scuffles began to break out, and officers wearing helmets moved into the crowd. Video showed officers overwhelming some of the protesters, pushing them to...