Death to the French (1932) aka Rifleman Dodd.

From the author of The African Queen and the Hornblower series, comes a thrilling novel of guerrilla warfare against Napoleon's troops. In 1810, with Wellington's army penned behind the Tigus, Rifleman Dodd becomes separated from his regiment. When he stumbles upon a band of Portuguese guerrillas, he transforms this ramshackle group into an organized fighting force that continually harries the infuriated enemy. A rousing war and adventure story.