The sentence makes sense. I'm not sure of the rule, but the someone could be considered the raiding parties. And actually I would use to prevent without needing to prevent sb. I speak u.s. English so it could depend on dialect.

I think you're right. You could see the sentence as being as shorter way of saying "His patrols prevented the raiding parties (from happening/succeeding/occurring)", but I think many people would feel there should be something else after the sentence, because of the word "preventing", which implies a "from..." clause.