Semper Fi --- Trying to locate a book

I can't remember the name of it but the premise is this:

Two portals open on a typical fantasy world. A large stockpile of modern military weaponry and instructions to use it fall into the evil kingdom. A US Marines Drill Instructor (or maybe Gunnery Sergeant) falls through to the good kingdom. He proceed to train them in geurilla tactics to overthrow the evil empire.

Any help would be greatly appreciated.