It’s fiction but if you’re interested in alternate history I really enjoyed Clash of Eagles by Alan Smale. The synopsis is below, but it doesn’t mention the twist: the author was inspired by the Mississippian mound culture and the idea that maybe just maybe they built them as platforms to jump from…and fly/glide! Hence their military prowess in this series:

Perfect for fans of action-adventure and historical fiction—including novels by such authors as Bernard Cornwell, Steve Berry, Naomi Novik, and Harry Turtledove—this stunning work of alternate history imagines a world in which the Roman Empire has not fallen and the North American continent has just been discovered. In the year 1218 AD, transported by Norse longboats, a Roman legion crosses the great ocean, enters an endless wilderness, and faces a cataclysmic clash of worlds, cultures, and warriors.

Ever hungry for land and gold, the Emperor has sent Praetor Gaius Marcellinus and the 33rd Roman Legion into the newly discovered lands of North America. Marcellinus and his men expect easy victory over the native inhabitants, but on the shores of a vast river the Legion clashes with a unique civilization armed with weapons and strategies no Roman has ever imagined.

Forced to watch his vaunted force massacred by a surprisingly tenacious enemy, Marcellinus is spared by his captors and kept alive for his military knowledge. As he recovers and learns more about these proud people, he can’t help but be drawn into their society, forming an uneasy friendship with the denizens of the city-state of Cahokia. But threats—both Roman and Native—promise to assail his newfound kin, and Marcellinus will struggle to keep the peace while the rest of the continent surges toward certain conflict.