Dux

Dux (/dʌks, dʊks/, pl.: ducēs) is Latin for "leader" (from the noun dux, ducis, "leader, general") and later for duke and its variant forms (doge, duce, etc.). During the Roman Republic and for the first centuries of the Roman Empire, dux could refer to anyone who commanded troops, both Roman generals and foreign leaders, but was not a formal military rank.

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.

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.

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.^{[citation needed]} The Dux Belgicae secundae ("commander of the second Belgic province") is an example.