Imagine a world in which this magazine never existed. In that version of existence, the chief resident at New York's biggest hospital is a homeopath, I'm selling aluminum siding and right now you're reading the Daily Racing Form.

This musing has been a very small exercise in what's called alternate history, or alt history, defined in its Wikipedia entry as “a genre of fiction consisting of stories that are set in worlds in which one or more historical events unfolds differently from how it did in reality.”

For example, in Michael Chabon's novel The Yiddish Policemen's Union, Alaska rather than Israel is home to millions of the world's Jews displaced by World War II. Tensions exist between the immigrants and the native Tlingit, but everyone likes salmon. In Quentin Tarantino's movie Inglourious Basterds, a special-forces team kills Hitler, who still couldn't make it as a painter. In Philip K. Dick's book The Man in the High Castle, the Axis powers have won World War II, and a character in the book is writing an alternate-history novel in which the Allies won the war. Philip K. Dick did things like that.

Of course, alt history contemplates themes other than World War II and its aftermath. Another popular scenario is the one in which the South wins the Civil War. In such an America, black men might be incarcerated at a rate seven times higher than that of white men, or a southerly-facing Congress could forbid academic scientists from advising the Environmental Protection Agency about their research. Ahem.

To this list of alt-history works, add the new novel Clash of Eagles, by Alan Smale. He's an astrophysicist at the nasa Goddard Space Flight Center in Maryland, but his book is on terra firma if a bit incognita. It contemplates a world in which the Roman Empire has continued chugging along, unfallen, into the early 13th century. Having made Scandinavia a vassal state, Rome has a navy of Norse longships. And in 1218 those ships carry to the eastern shore of North America the 33rd Legion, which commences to march west under its eagle standard.

The Roman invaders figure they are going to have an easy time with any locals, until they reach Cahokia, the major city of the Mississippian culture, site of some 120 minor earthen mounds and one absolutely enormous one, now known as Monk's mound. Yes, Cahokia really existed.

“I used to go for family vacations at Hadrian's Wall when I was much younger,” Smale says. “So I've been interested in the Romans for a long, long time. More recently, I was reading Charles Mann's 1491, about the Americas before the Columbus voyage, and he has a large section on Cahokia, and I got fascinated with it.” As Scientific American's sister publication Nature recently noted in a report about evidence for a huge flood there some eight centuries ago, “Cahokia was a pretty big deal in the 1100s.”

To give them a fighting chance against a steel-bearing, combat-hardened legion, Smale's Cahokians have come up with a rather advanced technology for the time. (And here's your requisite spoiler alert.) His research alerted him to “the dominance of flying imagery in Mississippian art. It was clear that they had a birdman cult. And that really got me thinking about what is the point of having these big, tall mounds if you're not going to throw yourself off them. And so the Mississippian culture in the book has developed a form of flying. Essentially they have things made of wood and deer skin that are very much like hang gliders.”

With air power and incendiary devices, the Cahokians are, as Smale puts it, “able to give the Romans a run for their money.” And their raptor imagery versus the Roman standard provides the clash of eagles of the title.

The book is the first volume of a trilogy that will eventually take the reader farther west and south. The saga will presumably wrap up before Robert E. Lee accepts the surrender of William Tiberius Sherman in Atlanta.