legion

legion – n - A division of the Roman army, usually comprising 3000 to 6000 soldiers.

For centuries Rome had the best-trained, most efficient army the world had ever seen. They usually won, but there were at least two notable exceptions: in A.D. 9, Varus walked into a German ambush and lost three legions, and in A.D. 378 the emperor Valens took over 60% casualties against the Visigoths at Adrianople.

Jerry Pournelle noted that there have been only two extended periods of peace in history, the Pax Romana and the Pax Brittanica, and both were enforced by imperial military forces.