The author has determined in an earlier McFarland book (The Historic King Arthur, 1996, paperback 2007) that there was not a historic King Arthur during the sixth century. However, as listed in The Historia Brittonum, there was a "great king of all the kings of Britain" named Ambrosius Aurelianus who was conflated with a heroic Arthur of the second century, and hence with the legendary King Arthur. To further authenticate the Celtic/Romano "King Arthur,"—that is, Ambrosius—the author here examines seven major historical figures of the period A.D. 383-500 based upon the Genealogical Preface of The Anglo-Saxon Chronicle and the emendation of dates in that chronicle. Those seven allies and adversaries are Vortigern, Vortimer, Vitalinus, Cunedda, Cerdic, Octha, and Mordred.

Through an extensive analysis of Arthur's 12 battles listed in the Historia Brittonum, this work explores both the influences of the High King's allies, and the shifting allegiances of his enemies. A battle list provides possible geographic locations for each of the battles, including a new site for Arthur's fateful battle at Camlann.