Comes Britanniarum was a military post in Roman Britain

Roman Britain

Roman Britain was the part of the island of Great Britain controlled by the Roman Empire from AD 43 until ca. AD 410.The Romans referred to the imperial province as Britannia, which eventually comprised all of the island of Great Britain south of the fluid frontier with Caledonia...

, with command of the mobile field army from the mid 4th century onwards.

It is listed in the Notitia Dignitatum

Notitia Dignitatum

The Notitia Dignitatum is a unique document of the Roman imperial chanceries. One of the very few surviving documents of Roman government, it details the administrative organisation of the eastern and western empires, listing several thousand offices from the imperial court down to the provincial...

as being one of the three commands in Britain, along with the Dux Britanniarum

Dux Britanniarum

Dux Britanniarum was a military post in Roman Britain, probably created by Diocletian or Constantine I during the late third or early fourth century....

 and Comes litoris Saxonici. His troops were the main field army in Britain or comitatenses

Comitatenses

Comitatenses is the Latin plural of comitatensis, originally the adjective derived from comitatus , itself rooting in Comes .However, historically it became the accepted name for...

 and not the limitanei

Limitanei

The limitanei, meaning "the soldiers in frontier districts" The limitanei, meaning "the soldiers in frontier districts" The limitanei, meaning "the soldiers in frontier districts" (from the Latin phrase limes, denoting the military districts of the frontier provinces established in the late third...

 or frontier guard commanded by the other two.

Some historians formerly considered the post to have been introduced after 410 during a now-discounted re-occupation of Britain under Honorius.

The first comes

Comes

Comes , plural comites , is the Latin word for companion, either individually or as a member of a collective known as comitatus, especially the suite of a magnate, in some cases large and/or formal enough to have a specific name, such as a cohors amicorum. The word comes derives from com- "with" +...

in Britain was Gratian the Elder

Gratian the Elder

Gratianus Funarius, also known as Gratianus the Elder or Gratianus Major , was an Illyrian soldier of the Roman Empire who flourished in the 4th century...

, the father of emperor Valentinian I

Valentinian I

Valentinian I , also known as Valentinian the Great, was Roman emperor from 364 to 375. Upon becoming emperor he made his brother Valens his co-emperor, giving him rule of the eastern provinces while Valentinian retained the west....

, who commanded the British field army (comitatus

Comitatenses

Comitatenses is the Latin plural of comitatensis, originally the adjective derived from comitatus , itself rooting in Comes .However, historically it became the accepted name for...

) holding this title. It seems to have been an appointment during some unrecorded crisis at the time.

A permanent office was created in the late fourth or early fifth century, perhaps by Stilicho

Stilicho

Flavius Stilicho was a high-ranking general , Patrician and Consul of the Western Roman Empire, notably of Vandal birth. Despised by the Roman population for his Germanic ancestry and Arian beliefs, Stilicho was in 408 executed along with his wife and son...

 who withdrew troops from Britain to defend Italy in 402. Alternatively, it may have been instituted by Magnus Maximus

Magnus Maximus

Magnus Maximus , also known as Maximianus and Macsen Wledig in Welsh, was Western Roman Emperor from 383 to 388. As commander of Britain, he usurped the throne against Emperor Gratian in 383...

 or Constantine III

Constantine III (usurper)

Flavius Claudius Constantinus, known in English as Constantine III was a Roman general who declared himself Western Roman Emperor in Britannia in 407 and established himself in Gaul. Recognised by the Emperor Honorius in 409, collapsing support and military setbacks saw him abdicate in 411...

.
According to the Notitia Dignitatum the comes commanded six cavalry and three infantry units, probably a force of no more than 6,000 troops. This tiny force was charged with supporting the frontier troops in fending off the increasing number of barbarian raids during the period. Some units seem to have been transferred from the Duke of Britain's or Count of the Saxon Shore's armies. The office was not in place for long as the last Roman troops are recorded in Britain no later than 409.