Kimi to Boku no Saigo no Senjou, Aruiwa Sekai ga Hajimaru Seisen

For more than a century, the technologically powerful Empire and the magic-wielding Nebulis Sovereignty have fought without end. Iska, one of the Empire's youngest elite swordsmen, is released from prison on one condition: defeat the feared Ice Calamity Witch. That enemy is Aliceliese Lou Nebulis XI, princess of the Sovereignty and a mage whose power rivals his own. When they finally meet, neither finds the monster their country described. Iska and Alice remain loyal to opposing nations and continue clashing on the battlefield, yet they discover the same tastes, frustrations, and desire to end the war. The more they understand each other, the more dangerous every encounter becomes, because peace between them could be treated as betrayal by both sides.