On August 15, 2014, an angry 20-something ex-boyfriend published a 9,425-word screed and set in motion a series of vile events that changed the way we fight online. The post, which exhaustively documented the last weeks of his breakup with the video game designer Zoë Quinn, was annotated and punctuated with screenshots of their private digital correspondence — emails, Facebook messages and texts detailing fights and rehashing sexual histories. It was a manic, all-caps rant made to go viral.

And it did. The ex-boyfriend’s claims were picked up by users on Reddit and 4chan and the abuse began. Ms. Quinn and her immediate family members were threatened. Her private information was exposed, including old nude photos from a past relationship. Chat rooms popped up to discuss the best ways to “ruin her life” and fantasize about elaborate ways of killing her.