Juan_de_la_Corte_-_Battle_Scene_with_a_Roman_Army_Besieging_a_Large_City_-_WGA05366

A thought experiment on Reddit is on its way to becoming a major motion picture. Alyson Sheppard tries to answer the question:

Historian Goldsworthy says the MEU would probably lose in the long term—without the ability to resupply their modern weapons, they simply wouldn’t be able to overcome the Roman numbers. However, he says, they could destabilize the Roman Empire, encourage civil war, and initiate regional fracturing. "[The Marines] might discredit the Emperor by defeating the closest army to Rome," he says. "But they would lack the numbers to control Rome itself—with a population of a million or so—let alone the wider empire." 

(Photo: "Battle Scene with a Roman Army Besieging a Large City" by Juan de la Corte)