I wrote a brief history of the Mantu, based on the three sources in which they're referenced: Fleet Action, Heart of the Tiger and the ICIS Manual. I hope this helps. Please feel free to post it anywhere the Mantu question comes up.
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The major conflict between the Empire of Kilrah and the Mantu began around 2429. Terran historians believe that it was borne out of Kilrathi aggression towards civilian Mantu ships. The Mantu, an paranoid, insular race of explorers and prospectors, reacted to these attacks with extreme prejudice, pursuing a major invasion of Kilrathi space. It was warfare on a scale unprecedented and overwhelming to the Kilrathi of the 25th century. The conflict ultimately ended around 2431 when the Mantu withdrew their forces to pursue warfare in another theater.
Details of the Mantu’s ‘other war’ are scarce, save that it lasted some 235 years and concluded in 2666 with a victory on their part. In the years that followed their war, the Kilrathi demonized the Mantu as per their own sociological imperative. The Cult of Sivar religion, founded on the Prophecy of Sivar’s threat that the Kilrathi would be annihilated by their Star Gods should they ever falter in battle, would not allow for a conflict to be so ignobly ended. The Kilrathi came to believe that the war was not over but rather that the Mantu were constantly preparing for a second attack. In reality, the Mantu had offered no such threads and have never shown any practical interest in continuing the war with the Kilrathi.
This belief, however, gained even more strength among the Kilrathi in 2598 when a small border skirmish broke out between the two nations. The battle, instigated by a Kilrathi attempt to cross the Mantu border, ended in a draw but it further reinforced Kilrathi paranoia regarding their former enemies. During the fighting the Kilrathi learned that Mantu technology had advanced more quickly than had their own, creating an even greater impetus to prepare for the questionable threat of a future Mantu invasion.
In 2667 a long range probe informed the Kilrathi leadership that the Mantu had finally completed their centuries old war with a neighboring race. Two years later, similar reports claimed they were reapportioning their forces. At the height of the Terran-Kilrathi War, an Empire truly convinced it was threatened on two fronts prepared for the worst…
