In 2431 Mantu explorers reached the Kilrah System -- where they are attacked by a Kilrathi. The Mantu, paranoid explorers with comparatively powerful technology, fend off the Kilrathi in a short war. Based in part on the stipulation that they always be victorious in their 'Star God' mythology, the Kilrathi demonize the Mantu as being an evil race which will return someday to fight them off. The Mantu, for their part, are engaged in another, more important, war (possibly with the Nephilim) on the other side of their territory. In 2598 the Mantu and the Kilrathi (having expanded) fight another skirmish, which ends in a draw.
Action Stations mentions a Confed probe which has discovered a 'warlike empire' 10,000 light years away from the Confederation -- which is a nod to the Nephilim rather than the Mantu, as the Mantu are *not* a warlike empire.
(Externally, the Mantu were created in 'Fleet Action' when Origin told Forstchen the Kilrathi would eventually be replaced by a new species. The ICIS manual for Prophecy confirmed that the Mantu were *not* the evil, Kilrathi-replacement race Forstchen envisioned... and so Action Station talks about the Nep's instead.)