Sharpe's Enemy by Bernard Cornwell

Sharpe's Enemy: Richard Sharpe and the Defense of Portugal, Christmas 1812 is a historical novel by Bernard Cornwell set during the Napoleonic Wars. Sharpe's Enemy was the sixth book in the Sharpe series written by Cornwell, but after the addition of many more novels to the successful series it resides about two thirds of the way through the series and Sharpe's military career.