If This Be Love — Elizabeth Renier

On her 18th birthday, beautiful Caroline Penwarden seemed to be standing on the threshold of all the happiness in England-secure in her beloved Cornwall, and in the love of Timothy Brencombe.
But the sudden arrival of Miles Courtenay, a rich young Birginian who had come to plead the Rebels' cause, plunged her into a dilemma of loyalty and intrigue, and shattered the foundation of her betrothal.