Ponyboy tells Cherry what happened to Johnny in chapter 2. It's a horrific and sobering flashback within a book that is already brutally honest about violence. Johnny was jumped by a group of Socs and beaten so badly that the Greasers thought that Johnny was dead.
His white T-shirt was splattered with blood. I just stood there, trembling with sudden cold. I thought he might be dead; surely nobody could be beaten like that and live.
We are told that the Greasers are used to seeing Johnny banged up because he gets beaten at home, and he's good in a rumble; however, this time it was so far beyond what any of them had ever seen.
Johnny recounts to the Greasers what happened to him, and he can barely get it out through the sobs. He's physically scarred by the encounter, but he has been mentally and emotionally broken by the event as well. Johnny will never be the same again. The Socs scared him that badly.
"A blue Mustang full . . . I got so scared . . ." He tried to swear, but suddenly started crying, fighting to control himself, then sobbing all the more because he couldn't. I had seen Johnny take a whipping with a two-by-four from his old man and never let out a whimper. That made it worse to see him break now.
Johnny is forever changed by the incident because of how viscerally brutal the attack was. He never walks alone after that, and he never goes anywhere without his switchblade. Ponyboy tells his readers that nobody would ever get the jump on Johnny like that again. Johnny would use the blade protecting himself or die trying.
And Johnny, who was the most law-abiding of us, now carried in his back pocket a six-inch switchblade. He'd use it, too, if he ever got jumped again. They had scared him that much. He would kill the next person who jumped him. Nobody was ever going to beat him like that again. Not over his dead body
What's great about the end of that flashback is how it foreshadows Johnny's usage of the blade in the coming chapters to protect himself and Ponyboy from getting attacked by another group of Socs.