Seasons of War (The Unbroken Saga Volume 1) by Lindsay French:

Sergeant Zayne Riven was born to take his father's place as the emperor's right hand man, but he has plans of his own. Trapped inside the corrupt regime he wishes to dismantle, Zayne plays loyal war hero while secretly fighting both sides of the war. He is determined to rise to the top and destroy the emperor, even if it means destroying his soul along the way. That is until one political prisoner shatters all of his plans. The emperor has captured the rebel woman Zayne once secretly saved. The woman he could never stop thinking about. And now Zayne has been ordered to break her spirit and use her to crush the uprising.

Kira Stiles is a true revolutionary. She papers alleyways with forbidden news and hijacks the emperor's magi-tech to write messages of rebellion across the sky. And she would die a thousand deaths before helping the emperor destroy the rebels. Captivity will not break her, not even at the hands of the grumpy and brooding Sergeant Riven. But as her imprisonment stretches on, Kira begins to suspect that the infamous war hero Zayne Riven may not the heartless loyalist he seems. He may secretly be as dangerous to the emperor as she is.

Zayne faces an impossible choice. Her life or the fate of the entire rebellion.