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.
[2] 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.
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.
[note 1] Under Diocletion, during the Tetrarchy, a new office called dux was created with powers split from the role of the governor of a province.
[citation needed] The office of dux was, in turn, made subject to the magister militum of his respective praetorian prefecture, and above him to the emperor.
In the Komnenian period, the title of doux replaced altogether the strategos in designating the military official in charge of a thema.
Dux is also the root of various high feudal noble titles of peerage rank, such as the English duke, the French duc, the Spanish and Portuguese duque, the Venetian doge, the Italian duca and duce, and the Byzantine Greek dukas or doukas (Gr.