Why Fritz Lang?
I've read in a couple of sources that Fritz Lang took the assignment to direct this because he needed the work and later tried to disown it. I think the film is a good one, up to a point, as an account of what life was like on the ground for Filipinos living under Japanese occupation and for Americans and guerrillas trying to accomplish their missions while evading discovery and capture. However, it could have been much better directed. The story, based on a true account, never quite coheres into a compelling narrative and the actors (and extras) often seem like they're going through the motions. There's never any real passion on display. While I like Fritz Lang's work, he tended to thrive more in a studio setting where he could control the look and feel and tone of a film and supervise the actors more closely to get what he wanted out of them. (Check out his numerous noir titles of the 1940s, e.g. SCARLET STREET, MINISTRY OF FEAR, etc.) He's clearly out of his element here, on location in a jungle setting where you had to adapt to the weather and climate conditions and in a situation where he had to direct a lot of non-professional locals and military personnel who were appearing on camera in key roles and as extras. I can't imagine that he was ever able to develop any patience with them.
So I wonder why Darryl Zanuck, then head of production at Fox, didn't hire a director better suited to the material, e.g. Henry King or Henry Hathaway, both of whom had ample experience shooting on location and dealing with non-professionals and both of whom were under contract to Fox at the time. There were plenty of other directors at Fox who were better alternatives than Lang, including Delmer Daves, Robert Wise, Anatole Litvak and Jean Negulesco, to name a few. Negulesco's THREE CAME HOME, made the same year, dealt with similar wartime material in as powerful a fashion as was possible at the time. It remains a remarkable film.
I will say this for AMERICAN GUERRILLA. Even though the war had been over for five years, it doesn't soften the depiction of Japanese brutality toward the Filipinos during their occupation. We don't see the full extent of the atrocities committed, of course, but we do see cold-blooded murder of civilians.