Manga: Seisenki Elna Saga

A fantasy manga by Shouko Tsutsumi.

Long ago, a hero fought and defeated a powerful monster that was terrifying the world. However, the beast would not die, and its wounds expelled a powerful wind that threatened to scour all life from the world. Climbing to the top of a mountain, the hero planted his sword in its peak. The edge of the blade diverted the wind, allowing the peaceful valley of Gimrey to form.

Fast forward to the present and the kingdom of Asaturl, whose royal line is descended from the hero. Unfortunately, they're not acting very heroic at the moment, waging a bloody war of conquest against their neighbors Ansas and Gudlang. With the war at a stalemate and threatening to bankrupt Asaturl, its Evil Chancellor comes up with a plan: Send Princess Elna, the last heir of the ancient hero and the only one who can remove the sword, to the top of the mountain, then use the threat of her removing its protection to force their enemies to submit to Asaturl. Caught between loyalty to her country and the prospect of being a tool for genocide, Elna reluctantly agrees.

Cut to the powerful Magic Knight, Prince Thjalfi of Ansas. Catching wind of the plot, he undertakes a solo mission into Asaturl's castle to kill Elna. Coming face-to-face with the beautiful, headstrong princess that not only doesn't fear him but half believes he should go ahead and strike her down, he's unable to deliver the killing stroke and abducts her instead. Thus begins the adventure and romance between a reckless young prince and a princess that two countries want killed on sight...