The Great War was a relatively brief but brutal conflict between the Kyrians and the Vaskans over their mutual homeworld, beginning in late 2374. The Kyrians were defeated with millions of casualties, and their race was relegated to second-class status for the next seven centuries.
The Kyrians have long believed that the Vaskans were the aggressors in the Great War, who desired Kyrian territories and resources. According to that history, Ambassador Daleth of the Vaskans entered an agreement with the warship Voyager, commanded by the ruthless Captain Kathryn Janeway, in which she would attack the Kyrians with her superior weaponry in exchange for the location of a cyclic wormhole. Janeway upheld her end of the deal by bombarding Kyrian population centers with biogenic weapons and executing the Kyrian leader Tedran; although this was far in excess of what the Vaskans had wanted- the Vaskans simply wanting Tedran eliminated rather than his people as a whole-, the Kyrians were then no match for the Vaskan forces.
In the 31st century, the EMH backup module was discovered beneath the ruins of Kesef. The Doctor detailed a much different encounter with the USS Voyager, which had stopped to trade medical supplies to the Vaskans in exchange for dilithium, Tedran being a raider who attempted to attack Voyager in the belief that they were helping the Vaskans and being killed by the Vaskan ambassador. During the negotiations, the Kyrians, led by Tedran, attacked to steal technology. Tedran was killed by Daleth after having taken Voyager crew members as hostages. Shortly after, the war began and Voyager was forced to retreat from a Kyrian fleet.
The Vaskans had never fully accepted the Kyrians' version of events, and with the Doctor's testimony, tensions rose to the point that another war threatened between the two species. Fortunately, the Doctor and Quarren were able to defuse the situation with solid evidence of what truly happened. (VOY: "Living Witness")