Here’s one way to make it as a screenwriter: apply online.

Warner Bros. has preemptively purchased the high-concept pitch “Rome, Sweet Rome” from first-time scribe James Erwin, an author and two-time “Jeopardy!” champion from Des Moines, Iowa.

Erwin’s pitch sale came about as a result of several postings on the website Reddit.com, an online community and social news aggregator where users vote to determine which posts land on the site’s home page.

Erwin set out to answer the question: “What if a unit of current U.S. Marines are suddenly transported back to ancient Rome and forced to do battle with the Roman legions?” Pic will follow the Marines as they’re flung into the past where they encounter one of the world’s most legendary villains and disrupt history. To return home, they have to set history back on the track they altered.

Madhouse Entertainment’s Adam Kolbrenner spotted Erwin’s “Rome, Sweet Rome” posts once they reached the top of Reddit and moved quickly to contact the writer and begin working with him to develop the concept. When it came time to find a home for the project, Kolbrenner brought it to WB’s Chris Gary, a young exec who encouraged the studio to move aggressively to acquire it.

Kolbrenner will produce with Gianni Nunnari (“Immortals”) of Hollywood Gang, where John Ridley will oversee the project for the company. Madhouse’s Robyn Meisinger will exec produce and Gary will shepherd the project for Warners.

Erwin’s next book, due in 2012, chronicles the history of U.S. military actions. He’s repped by Madhouse Entertainment and attorney Bruce Gellman.