Bent Tree Bride by Denise Weimer

Susanna Moore can’t get him out of her mind—the learned lieutenant who delivered the commission from Andrew Jackson making her father colonel of the Cherokee Regiment. But the next time she sees Lieutenant Sam Hicks, he’s leading a string of prisoners into a frontier fort, and he’s wearing the garb of a Cherokee scout rather than the suit of a white gentleman.
As both Susanna’s father and Sam’s commanding officer, Colonel Moore couldn’t have made his directive to stay away from his daughter clearer to Sam. He wants a better match for Susanna—like the stuffy doctor who escorted her to Creek Territory. Then a suspected spy forces Moore to rely on Sam for military intelligence and Susanna’s protection, making it impossible for either to guard their heart.