I think the fallacy here is that anything at all that changed back BC would alter the course of history such that there might not even be a United States of America, let alone a modern USMC. I think Alternate Universe plots are more acceptable. One of the best ones, I believe, was “Gunpowder God”, by H. Beam Piper IIRC. The central character is a Pennsylvania State trooper, a real outdoorsman, who stumbles into some kind of wormhole while out hunting woodchucks, and find that his surroundings are much the same, but populated by humans in a sort of early middle ages situation. He signs on with one of the competing factions, teaches the art of manufacturing black powder, soon becomes a favorite of the local female warlord . . . well, it was a good story and I’d enjoy reading it again.
lol