James Erwin was browsing Reddit on his lunch break when a thread piqued his interest. A user called The_Quiet_Earth had posed the question: "Could I destroy the entire Roman Empire during the reign of Augustus if I traveled back in time with a modern U.S. Marine infantry battalion or MEU?"

The question struck a chord with the 37-year-old Erwin, a technical writer from Des Moines, Iowa, who happened to be finishing a book called The Encyclopedia of U.S. Military Actions (Through Facts on File). Erwin tells PM that he wasn't impressed by other users' early attempts to answer this question, and so, posting under the username Prufrock451, he came up with his own response. Erwin wrote a 350-word short story chronicling the fictitious 35th Marine Expeditionary Unit (MEU), which suddenly disappears from modern-day Kabul and reappears on the Tiber River in 23 B.C. Erwin posted the piece, finished his meal, and went back to work.

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After work, Erwin checked reddit. Thousands of users had read his post and they demanded more. Excited and overwhelmed, Erwin continued submitting pieces of this growing Internet phenomenon. The next day, Los Angeles–based management firm Madhouse Entertainment contacted him about representation. Within the week, after Erwin had put just more than 3500 words to screen, Warner Brothers Studios bought the movie rights.

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Erwin's story, which he titled Rome, Sweet Rome, has a cult following among reddit members, its own subreddit on the site, and has inspired fan music and art. But from the beginning, his posts received comments critiquing the accuracy of his conjured tale. Other redditors commented. Historians commented. Marines commented. "You can definitely tell that the story was something that I dashed out on my lunch hour without doing a lot of research beforehand," says Erwin, an encyclopedia writer and two-time Jeopardy! champ. "Any Marine is going to see mistakes in it, and I'm sure if there were Romans around, they'd say the same thing." He plans on doing intensive technical research during the screenwriting process.

So—disregarding troubling questions about time travel and just why some temporally displaced Marines would feel compelled to destroy an empire——could a single MEU destroy the Roman Empire? To sort through the flood of online responses, PM talked to a Roman military expert and found out how the two sides would line up.

Infantry

An MEU typically contains about 2200 troops, along with their artillery and vehicles. According to Erwin's original reddit story (which will be altered for the movie), the Marines are transported back in time with what they have with them, including M1 Abrams battle tanks, bulletproof vests, M4 rifles, and grenades.

The year Erwin chose (23 B.C.) falls in the reign of Augustus, great-nephew of Julius Caesar and considered the first Roman emperor. His legions numbered nearly 330,000 men. They wore heavy leather and metal armor, carried swords and javelins, and operated catapults. They would have never heard the sound of an explosion before. "Obviously, there is a massive difference in firepower," says Roman military expert and author Adrian Goldsworthy. "Not only would Roman armor be useless against a rifle round—let alone a grenade launcher or a .50 caliber machine gun—it would probably distort the bullet's shape and make the wound worse."

Trajan's Column, Rome Getty

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In the reddit story, however, Erwin said the Marines would not be resupplied with bullets, batteries, or gasoline from the modern world. "There would be no way of obtaining replacements for these supplies in the ancient world," Goldsworthy says. "An average unit of Marines is not likely to be able to make an oil refinery, start generating electricity, or create machine tools to make spare parts for equipment." And even if they could figure it out, it would take many months or even years. So, as soon as the Marines ran out of gas, their tanks would become little more than hunks of metal.