The Menthar-Promellian War was a devastating conflict that erupted between the Menthar and Promellian species. The root cause of the war, at least according to the Federation, was believed to have been attributed to a Promellian starship that entered a holy Menthar system during a sacred time. This sparked off a catastrophic conflict between the two species which ultimately led to both of their destruction. (TNG novel: The Buried Age)