Senjou de Shoujo wa Karada o Kakeru

Protagonist is a battle hardened soldier who enjoying his vacation in some famous tropical resort country. But his times of peace doesn't last. There’s a swift and violent coup d’etat that leads our hero and his younger sister to be captured. Our hero’s exploits have caught the eye of the rebellious men, and they force him to join their cause and lead their troops while his sister is being held hostage. He goes along with their plot against his will, but who knows; perhaps he’ll discover the truth and feel that the rebels are on the side of right. Furthermore, it doesn’t hurt that there are a few attractive lady officers and mercenaries hanging around. How will their fates pan out in the fires of war?

[From White Masq]