John Harris 
So Far From God [EPUB ebook] 

With Europe on the brink of the First World War, Pierce Slattery, a renegade cavalry officer with the British Army, brings an astonishing insight and masterful fighting abilities to the aid of a revolution, led by Pancho Villa. Their army of ill-trained, poorly prepared peasants are fighting for their lives and their freedom — but British Intelligence has an interest in the Mexican Revolution and in the striking Slattery.