Hinowa ga CRUSH!

On the island of Wakoku, the civil war offers anyone the chance of becoming a hero -- from fishing women to bastard children, any can make a name for themselves by working through the rigorous ranks of the army. Hinowa, a young woman with extraordinary skills, is determined to do just that, but soon learns things may not always go to plan...More than ten years after a disastrous battle, the war is still ongoing, and a girl with black hair and black markings washes up on shore...

(Source: Yen Press)