Princess Warrior Masamune's Legend: Bontan!!

This manga presents an unusual take on the story of Date Masamune, starting in 1571. The Date family has a young daughter named Hagi, who is as lively and strong-spirited as a boy. She falls ill with small pox and barely survives, losing sight in her right eye in the process. Her parents take this opportunity to correct the mistake of her birth--the fact that she was born a girl. When she awakes, they insist to the young child that she is their oldest son, Bontenmaru (the childhood name of Masamune) and proceed to raise her as their son! Now the legendary life of a female Date Masamune begins!

(from mangaupdates)