“… in which Captain Sharpe has to protect a philandering diplomat and, deserted by his ally, faces the enemy” (as the blurb says), with the part of the philandering diplomat going to Ambassador Henry Wellesley, the Duke of Wellington’s younger brother, and Colonel Henri Vandal being the enemy sought by Sharpe and his men during the surprise British victory over the French at the Battle of Barrosa.
Author’s page: Sharpe’s Fury – Bernard Cornwell