Haken no Kouki Altina

Inept with a sword, unable to ride a horse, and apathetic toward the empire he serves, Regis Aurick is a hopeless soldier who spends his days buried in books. Banished to the borderlands, he encounters a stirring young woman with red hair and ruby eyes. She is Fourth Princess Marie Quatre Argentina de Belgaria, known simply to Regis as Altina. "...Do you want to become my tactician or not?" Having sought him for his rumored strategic prowess, she aspires to reform an empire led by self-seeking nobles and driven to pointless wars, the legendary blade of an emperor, the Grand Tonnerre Quatre, at her hip. Continuously underestimated by her regiment, including her would-be tactician, the princess resolves to prove her worth beyond a shadow of a doubt—no matter the risk. Here is a tale of war, love, and politics, woven by the bookworm and the sword princess.

(Source: J-Novel Club)