Our Last Crusade or the Rise of a New World, Vol. 2 (light novel) by Kei Sazane
As the century-long war between the Empire and the Nebulis Sovereignty rages on, the star-crossed lovers from enemy nations continue to miss their opportunity for a rematch. But when Iska and Aliceliese are both given orders from their respective superiors to track down the source of this ancient grudge, they might just have a chance to battle it out.