Charles Oman 
Routledge Revivals: A History of the Art of War in the Middle Ages (1978) [PDF ebook] 
Volume One 378-1278

First published in 1898, this history of medieval warfare, written by one of the great medievalists of his time, Sir Charles Oman, remains for students and general readers one of the best accounts of military art in the Middle Ages. The book begins with the significant battle of Adrianople in 378 A.D. (the most fearful defeat suffered by a Roman army since Cannae in 216 B.C.) and Marignano (1515 A.D.), the last of the triumphs of the medieval horseman. It was extensively revised and edited by John H. Beeler in 1953 to incorporate many new facts uncovered since the late nineteenth century. This edition is based on Methuen’s 1978 revised and enlarged edition, which includes new chapters and the author’s original preface.