Sharpe's Siege by Bernard Cornwell

Sharpe's Siege (Richard Sharpe's Adventure Series, No. 9) - Sharpe and his men are put under orders to Naval Captain Horace Bampfylde.  He is ordered to take the fort at Arcachon ambush the French supply road and then recon the populace morale in Bordeaux.  A French officer who is spying for the British brings orders for Sharpe to assist him in any way he needs.  When the navy drops Sharpe and his men at the landing point, Bampfylde tells Sharpe he is not needed and to go straight to the ambush. 

Of course Bampfylde screws up, Sharpe captures the fort as well as an American privateer, Cornelius Killick.