Since 1995, scientists have kept a watchful eye on the Ngogo chimpanzees in Uganda’s Kibale National Park. For around two decades, the wild apes coalesced as a unified group that mated and hunted together, although they divided themselves into three tighter-knit factions: the Western, Central and Eastern clusters.

However, on June 24, 2015, researchers witnessed an unusual change.

That day, Aaron Sandel, now a primatologist at the University of Texas at Austin, and his colleague John Mitani, a primatologist at the University of Michigan, watched as chimpanzees from the Western cluster approached members of the Central one. Instead of their usual mingling, “all hell broke loose,” Mitani tells Carl Zimmer at the New York Times.

Screaming and fighting ensued. The Western chimps ran away, but the Central ones chased them. “Nothing like that had been observed before,” Sandel tells Scientific American’s Jason P. Dinh.

After that confrontation, things got more violent—and two distinct groups eventually emerged, culminating in an ongoing “civil war” among the largest-known community of wild chimpanzees.