Fortune's Bride (Royal Dynasty #4)
Roberta Gellis

War was Robert Moreton’s passion and he had found a position as aide de camp to Sir Arthur Wellesley, who headed the British force assigned to drive the French out of Portugal. War requires beasts of burden. Robert was sent to collect horses, mules, and oxen. He was horrified to discover instead of mules in a seaside village a British maiden in distress.
That was the luckiest find of his life, but then control of the war was wrested from Sir Arthur’s hands. Stalemate under incompetent leaders was followed by a retreat that nearly cost the life Robert had saved and made wonderful.