The 1996 science fiction novel An Oblique Approach by Eric Flint and David Drake is available for free reading in a variety of eBook formats from the Baen Free Library. The book is the first in the Belisarius series, which puts the sixth century Byzantine general of that name in the middle of a war between two future societies:
In northern India the Malwa have created an empire of unexampled evil. Guided or possessed by an intelligence from beyond time, with new weapons, old treachery, and an implacable will to power, the Malwa will sweep over the whole Earth. Only three things stand between the Malwa and their plan of eternal domination: the empire of Rome in the East, Byzantium; a crystal with vision; and a man named Belisarius, the greatest commander Earth has ever known.
Books 2 through 4 in the series also can be read at the Free Library: