In Love and War — Mary Nichols

Married to an inveterate gambler who was prepared to dispose of her life in order to pay his debts, Louise Oakingham had fled … only to find herself torn between a husband she despised and a man she doubted.
Was Captain Paul Fourier, home on leave from the Peninsular Campaign, her jailor or her savior? Why, when he insisted that she accompany him to Lisbon, had he then left her alone in a strange city?
It was ironic that the safest place for her at that moment was in the middle of a war, and in deciding to go after the Captain, Louise began to understand just why women followed their men into battle…