Comes Britanniarum in the context of "Comitatenses"

Count Theodosius (Latin: Theodosius comes; died 376), Flavius Theodosius or Theodosius the Elder (Latin: Theodosius major), was a senior military officer serving Valentinian I (r. 364–375) and the Western Roman Empire during Late Antiquity. Under his command the Roman army defeated numerous threats, incursions, and usurpations. Theodosius was patriarch of the imperial Theodosian dynasty (r. 379–457) and father of the emperor Theodosius the Great (r. 379–395).

Appointed comes rei militaris per Britannias (commander of mobile military forces for the Diocese of the Britains) by Valentinian, Theodosius put down the Great Conspiracy (367–368) and the usurpation of Valentinus. After restoring order in Britain he returned to continental Europe and fought against the Alemanni; as Valentinian's magister equitum (Master of Horse) he successfully invaded Alemannic territory (371 or 370). In 372 Theodosius led a successful campaign against the Sarmatians. Within the same year Firmus, a Mauritanian prince, rebelled against Roman rule with the help of African tribes like the Abanni and Caprarienses. Theodosius was sent to Africa and in two hard-fought campaigns (373–374) put down the insurrection. In 376, after the death of emperor Valentinian, he was arrested and executed, presumably as he was seen as a threat to the new western emperors Gratian and Valentinian II.

Dux Britanniarum was a military post in Roman Britain, probably created by Emperor Diocletian or Constantine I during the late third or early fourth century. The Dux (literally, "(military) leader" was a senior officer in the late Roman army of the West in Britain. It is listed in the Notitia Dignitatum as being one of the three commands in Britain, along with the Comes Britanniarum and Count of the Saxon Shore.

His responsibilities covered the area along Hadrian's Wall, including the surrounding areas to the Humber estuary in the southeast of today's Yorkshire, Cumbria and Northumberland to the mountains of the Southern Pennines. The headquarters were in the city of Eboracum (York). The purpose of this buffer zone was to preserve the economically important and prosperous southeast of the island from attacks by the Picts (tribes of what are now the Scottish lowlands) and against the Scoti (Irish raiders).

The Count of the Saxon Shore for Britain (Latin: comes littoris Saxonici per Britanniam) was the head of the Saxon Shore military command of the later Roman Empire.

The post was possibly created during the reign of Constantine I, and was probably in existence by AD 367 when Nectaridus is elliptically referred to as such a leader by Ammianus Marcellinus. The Count's remit covered the southern and eastern coasts of Roman Britain during a period of increasing maritime raids from barbarian tribes outside the empire. The Count was one of three commands covering Britain at the time, along with the northern Dux Britanniarum and central Comes Britanniarum.