Aztec Gods

Martin Bragg is a Snapshot Explorer, flying into wild, unexplored alternate realities in search of adventure, treasure and video of strange cultures or animals he can sell to TV networks or big Internet companies. He doesn't want to be a conqueror or king, but he finds himself among modern US mercenaries trying to set themselves up as God kings of an Aztec empire that has survived almost unchanged into the modern age in a hidden alternate reality.
Modern weapons versus arrows and obsidian swords. It should be an even bigger mismatch than the historic conquest. The Aztecs already have Gods, though, and they are far more terrifying than airplanes and machine guns.