Sharpe's Battle by Bernard Cornwell

Sharpe's Battle (Richard Sharpe's Adventure Series #12) - Napoleon's Imperial army Brigadier General Guy Loup is waging war on the Spanish civilians.  When Richard Sharpe comes across two of the French soldiers killing and raping he summarily orders their execution.  Because of this Loup attacks the fort of San Isidro and takes revenge on a Portuguese  unit.  They meet one to one in the streets of Fuentes de Oñoro for their final battle.