Vermillion (Hundred Days #1)

On the eve of war, General Matthew Webb has returned to his division in Belgium. Upon arriving, he finds his garrison in disarray and the regimental surgeon, Doctor Addison, on his deathbed. With Napoleon on the horizon, Matthew struggles to prepare his men while feeling frustrated by the regiment's headstrong nurse.
Katherine Foster, trained as a doctor in America, has come with Doctor Addison to the battlefields of Europe in hopes of being allowed to practice. The illness of her mentor and the arrival of the taciturn General Webb conspire to dash her hopes, but Kate stubbornly refuses to be daunted.
Each resolving to overcome the other in a contest of wills, Matthew and Kate are thrown together when events inside the garrison take a dark turn.
It is an uneasy alliance, and for better or worse one they both discover they cannot live without.