Original usage Until the 3rd century,
dux was not a formal expression of rank within the Roman military or administrative hierarchy. In the
Roman army, 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. 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.
Change in usage By the mid-3rd 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 of the regular military formations. Such appointments were made to deal with specific military situations when the threat to be countered 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 they were invariably
Viri Perfectissimi, i.e., members of the second class of the
equestrian order. Thus, they would have out-ranked the commanders of provincial legions, who were usually
Viri Egregii – equestrians of the third class.
Duces differed from
praesides 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 3rd century that the term
dux emerged as a regular military rank held by a senior officer of
limitanei – i.e. frontier troops as opposed those attached to an Imperial field-army (
comitatenses) – with a defined geographic area of responsibility.
Diocletian's reforms Under Diocletian, during the
Tetrarchy, a new office called
dux was created with powers split from the role of the governor of a province. The dux was the highest military office within the province and commanded the legions, but the governor had to authorise the use of his powers after which the
dux could act independently and handle all military matters. The
Dux Belgicae secundae ("commander of the second Belgic province") is an example. Also the provinces were reorganised into
dioceses with each diocese administered by a
vicarius. As with the governors, the vicarius was assisted by a
dux. This
dux was superior to all other
duces within the dioceses; when the vicarius called the legions of the dioceses into action, all of the legions were at the command of the
dux. The office of
dux was, in turn, made subject to the
magister militum of his respective
praetorian prefecture, and above him to the
emperor. The
Dux per Gallias of the diocese of
Gaul is an example of this office. ==Byzantine Empire==