The Trespassers — Andrew J. Fenady

No mission is too dangerous as long as the cause--and the money--are right. Four soldiers of fortune, along with a beautiful woman, have crossed the Mexican border to dig up five million dollars in buried gold. But between the Trespassers and their treasure lie a merciless comanchero guerilla band, a tribe of hostile Yaqui Indians and Benito Juarez's army. It's a journey no one with any sense would hope to survive, or would even dare to try, except... The Trespassers