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.
In the years that followed, the Kilrathi demonized the Mantu per their own sociological imperative. The Cult of Sivar religion, founded on the Prophecy of Sivar's threat that the species would be annihilated by 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 both sides were preparing for a second, prologned fight. In reality, the Mantu had offered no such threats and have not since shown any practical interest in continuing the conflict.
This belief, however, gained even more strength in 2598 when a border skrimish broke out between the two nations. The battle ended in another draw. At the same time, it further reinforced Kilrathi paranoia regarding the Mantu: during the fighting it became clear that Mantu technology had advanced more quickly than that of the Kilrathi. Overlooked entirely by the Kilrathi was the fact that they had again instigated the fighting - in the two and a half centuries since the close of the war there has not been a single recorded border violation on the part of the Mantu.
Details of the Mantu's 'other war' are scarce, save that it involved a neighboring race or races and that it lasted some 235 years, finally ending in 2666. The next year a long range probe informed Kilrathi leadership that the centuries old conflict had concluded with a Mantu victory. Two years later, similar reports claimed forces were being reapportioned along the Mantu-Kilrathi border. So, at the height of the Terran-Kilrathi War, an Empire honestly convinced it was threatened on two fronts prepared for the worst...
