Dux Dux ( plural : " duces " ) is Latin for " leader " ( from the verb " ducere " , ' to lead ' ) and later for duke and its variant forms ( doge , duce , etc . ) . During the Roman Republic , " dux " could refer to anyone who commanded troops , including foreign leaders , but was not a formal military rank . In writing his commentaries on the Gallic Wars , Julius Caesar uses the term only for Celtic generals , with one exception for a Roman commander who held no official rank . Roman Empire . Original usage . Until the 3rd century , " dux " was not a formal expression of rank within the Roman military or administrative hierarchy . In the Roman military , a dux would be a general in charge of two or more legions . While the title of dux could refer to a consul or imperator , it usually refers to the Roman governor of the provinces . As the governor , the dux was both the highest civil official as well as the commander - in - chief of the legions garrisoned within the province . Change in usage . By the mid - third century AD it had acquired a more precise connotation defining the commander of an expeditionary force , usually made up of detachments ( i . e . " vexillationes " ) from one or more than one of the regular military formations . Such appointments were made to deal with specific military situations when the threat to be countered with seemed beyond the capabilities of the province - based military command structure that had characterised the Roman Army of the High Empire . From the time of Gallienus onwards for more than a century " duces " were invariably " Viri Perfectissimi " , I . e . members of the second class of the Equestrian Order . As such they would have out - ranked the commanders of provincial legions who were usually " Viri Egregii " - equestrians of the third class . " Duces " differed from " praeses " who were the supreme civil as well as military authority within their provinces in that the function of the former was purely military . However , the military authority of a " dux " was not necessarily confined to a single province and they do not seem to have been subject to the authority of the governor of the province in which they happened to be operating . It was not until the end of the third century that the term " dux " emerged as a regular military rank held by a senior officer of " limitaneii " - i . e . frontier troops as opposed those attached to an Imperial field - army ( " comitatenses " ) - with a defined geographic area of responsibility . The office under the Dominate . During the time of the Dominate , the powers of a " dux " were split from the role of the governor and were given to a new office called " dux " . The dux was now the highest military office within the province and commanded the legions , but the governor had to authorize the use of the " dux " ' s powers . But once authorized , the " dux " could act independently from the governor and handled all military matters . An example would be the Dux per Gallia Belgica " who was the dux of the province of " Gallia Belgica " . After Diocletian ' s Tetrarchy reform , the provinces were organized into dioceses each administered by a vicarius . As with the governors , the vicarius was assisted by a dux . This dux was superior to all of other duces within the dioceses and when the vicarius called the legions of the dioceses into action , all of the legions were at the dux ' s command . An example would be the Dux per Gallia who was the dux of the dioceses of Gaul . The office of dux was , in turn , made subject to the magister militum of his respective praetorian prefecture , and above him to the emperor . Later developments . In the Byzantine era of the Roman Empire , the position of dux survived ( Byzantine Greek : " δούξ " , " doux " , plural " δούκες " , " doukes " ) as a rank equivalent to a general ( " strategos " ) . In the late 10th and early 11th centuries , a " doux " or " katepano " was in charge of large circumscriptions consisting of several smaller " thema ta " and of the professional regiments ( " tagma ta " ) of the Byzantine army ( as opposed to the largely militia - like forces of most " themata " ) . In the Komnenian period , the title of " doux " replaced altogether the " strategos " in designating the military official in charge of a " thema " . In the Byzantine navy , " doukes " of the fleet appear in the 1070s , and the office of " megas doux " ( " grand duke " ) was created in the 1090s as the commander - in - chief of the entire navy . Post - Roman uses . King Arthur , in one of his earliest literary appearances , is described as " dux bellorum " ( " " dux " of battles " ) among the kings of the Romano - Britons in their wars against the Anglo - Saxons . A chronicle from St Martin ' s monastery in Cologne states that the monastery had been pillaged by the Saxons in 778 , but that it was rebuilt by an " Olgerus , dux Daniæ " ( who may have been the historical person around whom the myth of Ogier the Dane formed ) , with the help of Charlemagne . " Dux " is also the root of various high feudal noble titles of peerage rank , such as ( via the French " duc " ) the English " duke " , the Spanish and Portuguese " duque " , the Venetian " doge " the Italian " duca " and " duce " and the modern Greek " doukas " ( δούκας ) . Italian Fascist dictator Benito Mussolini used the title of " dux " ( and " duce " in Italian ) to represent his leadership . One fascist - motto was " DVX MEA LVX " in Latin letters , meaning " Duce is my light " or " Leader is my light " .