Meow Wars
A Usenet war that lasted roughly two years and consisted mostly of the word "meow".
It started in 1996 when students from Harvard began crossposting into alt.fan.karl-malden.nose, whose regulars replied in kind. Before long the participants were posting "meow" — in lines, in paragraphs, in entire messages — to groups that had nothing to do with any of it. Every outraged response in a new group pulled more people in, and crossposting let a single thread spill across dozens of hierarchies at once.
The Meow Wars are usually counted among Usenet's longest flame wars, and as an early case of organised trolling for its own sake. There was no point to win, only attention to drain. It petered out in 1998, mainly because everyone got bored, which remains how most online wars end.