Benford, Gregory. "Manassas, Again"
Divergence: c 200 BCE
What if: Rome developed a steam-driven machine gun.
Summary: Rome's former American colonies fight a civil war in the 19th century.
Published: In Isaac Asimov's Science Fiction Magazine, October 1991; What Might Have Been? Volume 3: Alternate Wars (eds. Gregory Benford and Martin H. Greenberg), q.v.; The Best Alternate History Stories of the 20th Century (eds. Harry Turtledove and Martin H. Greenberg), q.v.; and The Mammoth Book of Alternate Histories (eds. Ian Watson and Ian Whates), q.v.