Harvas Black-Robe had led a band of Halogai mercenaries on a campaign of destruction through the nation-state of Kubrat on Videssos' northern border. Krispos, fighting General Petronas' rebellion along his western border, watched with unease and then horror as Harvas attacked and sacked several border towns on the Videssos-side of the border, including Imbros. After Petronas was defeated and committed suicide rather than be captured, Krispos turned his army around and marched toward Kubrat.
The Halogai soldiers were heavily armed and well-trained; though they were usually outnumbered, they still fought the Videssians to a standstill. Harvas was also a very powerful mage, and he used magic to his advantage, striking Videssian commanders dead and setting up ambushes by hiding the location of his troops. The campaign in northern Videssos-southern Kubrat quickly turned to a war of attrition that took a couple years to complete.
Ultimately, Krispos forced the Halogai and their dangerous leader to Pliskavos, a fortress-town along the border with Khatrish. Using a combination of infantry and artillery power, along with a special weapon from the navy, as well as magic from Krispos' patron Tanilis, the Videssians successfully carried the town and ended the war. Harvas' body was never found, but he was presumed to be dead, burned without recognition, as there was no conceivable way he could have got out of Pliskavos.
Recalling the troubled years of his youth, as well as the history of the region stretching back several generations, Krispos used the opportunity offered Videssos by its full-occupation of Kubrat, and annexed the entire country, ending the war.
Harry Turtledove did not give this war a name. The term "Kubrat War" has been chosen for convenience.