Complex sentence: When Sara requests that Hogan take her to a Mexican camp, he agrees because he had previously arranged to help the Mexican revolutionaries attack the French garrison in exchange for a portion of the garrison's strongbox if they are successful.
Split into simpler sentences: When Sara requests that Hogan take her to a Mexican camp, he agrees. Hogan has already arranged to help the Mexican revolutionaries attack the French garrison, in exchange for a portion of the garrison's strongbox if they are successful.