Aisare Kouhi no Yuuutsu

Meika, a tomboyish girl ends up becoming the wife of her childhood friend, the emperor Seiran, through an unexpected turn of events. Meika had wanted to join the military to protect him, but by becoming his wife, she finds herself in the position of being protected instead. Furthermore, Seiran shows a different kind of masculinity than usual, which makes her feel out of sorts. Then, one day, movements that seem to be aimed at Seiran's life begin to take place in the palace...
https://www.cmoa.jp/title/315568/