Harry Turtledove has said that his life was shaped and molded by reading "Lest Darkness Fall" by L. Sprague De Camp. To my mind this first series by the master of alternate history is a time travel book based loosely upon that previous classic. Elements of one of Caesar's legions are transported forward in time to the Byzantine Empire. Only it's not the Byzantine Empire, it's the Videssos Empire in another universe where magic works. Aside from that, it's the Byzantine Empire written by a Byzantine historian. Aside from just surviving, the hero must prevent his new homeland from being overrun by barbarians "Lest Darkness Fall". For this reason I consider this the best time travel series to a fictitious universe ever written, as well as the best series about a Roman Legion. It is a must read for anyone interested in the Roman Empire, the Byzantine Empire, or time travel.
One of the many things which make this series interesting is that the heroes are from Caesar's Rome. Rome is young energetic vibrant and, most of all, expanding at this time. They are transported to the Byzantine/Videssos Empire while it is in a period of decline. (If it were the real Byzantine Empire, it would be the final decline, but we can still hope for a revival of the Videssos Empire.) The decadence brought by a thousand years of Empire is sharply contrasted with the vibrant patriotism of our heroes who remain unknowing that this is the destiny of their empire too.
This isn't just a military book where the legion moves from battle to battle. This is a much more realistic and complete world where the hero is thrust into the middle of court politics, and has to fight to overcome the shear inertia of the Empire's slide towards collapse. The hero spends more time facing corruption, political intrigue, distrustful monarchs, intolerant monks, and tax collectors than he does facing mounted cavalry units.
I don't want to repeat the excellent review about the warfare in this series written by Robert, 12 MAR 99, under "The Misplaced Legion" (Videssos Cycle, Book 1). I would like to add though, that this is not a series about a general. This is not a David Drake/S.M. Sterling series about Belisarius. As Robert points out, the hero of this series stands in the middle of the battles, and seldom knows more of what's going on than immediately to his left and right. The battles themselves are mostly standup fights where two sides hack at each other. This is really much more realistic though. Most battles, especially in the Roman era, were fought this way. Even though nowadays it seems like every book we read is about Belisarius, brilliant generals with innovative battlefield ideas come along only once in a thousand years or so. Most battles are fought without them.
"The Misplaced Legion" (Videssos Cycle, Book 1) is followed by "An Emperor for the Legion" (The Videssos Cycle Book, 2), "The Legion of Videssos" (Videssos Cycle, Book 3), and "Swords of the Legion" (Videssos Cycle, Book 4). There are two prequel series about Videssos. The first is the Tale of Krispos series, beginning with "Krispos Rising." This is actually a two book story, which is excellent, and a third follow on novel which is very good. The other prequel, The Time of Troubles series, begins with "The Stolen Throne". This series is entertaining, but not really as good as the first two series.
Although there are ten other books about Videssos, there can never be enough. And there are only four books about the Misplaced Legion. There needs to be a sequel series not another prequel. Perhaps "Legion of Videssos: Next Generation" where Marcus's son, born and raised in Videssos, can become Emperor, with Dad and his Legion helping out of course. This could lead to a new golden age for Videssos, and maybe even . . .