On March 11, an hour before the scheduled start of a Donald Trump rally in Chicago, I checked into the venue’s media pen—a rectangle of metal bike racks corralling the press—and introduced myself to a few of the journalists there. I was talking to Sopan Deb, a CBS News reporter who’s been following the Trump campaign for months, when the first protest erupted in the crowd. Trump supporters had encircled some guys wearing “Muslims United Against Donald Trump” T-shirts. Deb excused himself, hoisted his video camera onto his shoulder, and sprinted out of the pen for a better angle on the hubbub.

When he returned, he set down his camera and resumed chatting with me as though nothing had happened. A newspaper writer sidled over to join our pleasant conversation. Within a minute or two, another protester was yelling. My new pals swiveled their heads, quickly pinpointed the disturbance, and raced back into the crowd....

The flare-ups continued and intensified. Police swarmed into the stands. Everyone was on their feet, shouting and pointing. When a PA announcement informed us that the event had been canceled, the place detonated.

Reporters scurried to edges of the media pen, still hemmed in, and leaned their cameras over the barricades. Realizing this was folly, they soon mounted a jailbreak. I followed them and found myself amid a churning mob on the arena floor—scuffles everywhere, people shrieking at each other, ripping signs from hands, knocking hats from heads, shoving, stumbling. The cameras raced from one brawl to the next.