The Littlehorn Massacre occurred at Luna's School for Gifted Unicorns, in Littlehorn Valley. The massacre was caused when a zebra refugee convoy had to pass close to the school. The refugees were mistaken as soldiers by worried Equestrians, resulting in the school firing upon the convoy. The ponies in the school soon realized their mistakes, but casualties had already been sustained by the zebras.
This event lead to the abdication of Princess Celestia from the throne, giving rise to Princess Luna and the Ministry Mares. This was also a turning point for the ongoing conflict, as the Zebras believed Luna to be Nightmare Moon (an agent of the Stars, the mythological enemies of all living things), ending all possibilities of peace between the two nations,