The Great Psionic War took place half a million years prior to the 24th century. Several races across the Milky Way Galaxy were involved, including the Selakar and the two races that formed the Arret Empire. One race that fought back when the war spread to its territory had mastered telepathic illusions.
Federation Department of Temporal Investigations agent and former archeologist, Teresa Garcia, conjectured that the telepathic illusionists were, in fact, the Talosians. (DTI novel: Watching the Clock)