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Comes Britanniarum was a military post in Roman Britain, with command of the mobile field army from the mid 4th century onwards.
It is listed in the Notitia Dignitatum as being one of the three commands in Britain, along with the Dux Britanniarum and Comes litoris Saxonici. His troops were the main field army in Britain or comitatenses and not the limitanei or frontier guard commanded by the other two.
The first comes in Britain was Gratian the Elder, the father of emperor Valentinian I, who commanded the British field army (comitatus) 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 who withdrew troops from Britain to defend Italy in 402. Alternatively, it may have been instituted by Magnus Maximus or Constantine III.. Irrelevant of its origin, it appears the title was fleeting and did not remain for long, and certainly did not have the permanence of the Dux Britanniarum and the Comes litoris Saxonici.
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.