Much has been made of the 'dux erat bellorum' or 'leader of battles' title given to Arthur in the HISTORIA BRITTONUM. For the most part, scholars have been led astray into thinking this was an indication that Arthur held a true Roman rank (or one patterned after an earlier Roman rank). The majority hold to something like Dux Britanniarum, the military leader in the north of Britain. Early Welsh sources refer to Arthur as 'miles', 'soldier', and this has seemed to lend support to the the Dux Britanniarum idea.
If I'm right and Arthur is Cerdic of Wessex, another possibility presents itself. In the ANGLO-SAXON CHRONICLE, Cerdic first appears in the annal entry for the year 495 A.D. He is referred to as an aldorman, according to the translator Garmonsway "perhaps a translation of principes." The Latin version of the ASC uses duces duo when referring to Cerdic and Cynric.
Here is the definition of alderman or ealdorman as found in the Bosworth and Toller dictionary. The reader will note the meaning does include that of duke and often denoted a military leader. In my opinion, then, dux erat bellorum is merely a Latin rendering for ealdorman.
I. an elderman, ALDERMAN, senator, chief, duke, a nobleman of the highest rank, and holding an office inferior only to that of the king; mājor nātu, sĕnātor, prŏcer, princeps, prīmas, dux, præfectus, trĭbūnus, quīcunque est aliis grădu aut nātu mājor. The title of Ealdorman or Aldorman denoted civil as well as military pre-eminence. The word ealdor or aldor in Anglo-Saxon denotes princely dignity: in Beowulf it is used as a synonym for cyning, þeóden, and other words applied to royal personages. Like many other titles of rank in the various Teutonic languages, it, strictly speaking, implies age, though practically this idea does not survive in it any more than it does in the word Senior, the original of the feudal term Seigneur. Every shire had its ealdorman, who was the principal judicial officer of the shire, and also the leader of its armed force. The internal regulations of the shire, as well as its political relation to the whole kingdom, were under his immediate guidance and supervision,—the scír-geréfa, or sheriff, being little more than his deputy, and under his control. The dignity of the ealdorman was supported by lands within his district, which appear to have passed with the office,—hence the phrases, ðæs ealdormonnes lond, mearc, gemǽro, etc. which so often occur. The ealdorman had also a share of the fines and other monies levied to the king's use; though, as he was invariably appointed from among the higher nobles, he must always have possessed lands of his own to the extent of forty hides, v. Hist. Eliens. ii. 40. The ealdormen of the several shires seem to have been appointed by the king, with the assent of the higher nobles, if not of the whole witena gemót, and to have been taken from the most trustworthy, powerful, and wealthy of the nobles of the shire. The office and dignity of ealdorman was held for life,—though sometimes forfeited for treason and other grave offences; but it was not strictly hereditary