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The Comes Britanniarum (Latin for "Count of the Britains") was a military post in Roman Britain with command over the mobile field army from the mid-4th century onwards. It is listed in the List of Offices as being one of the three commands in Britain, along with the Duke of the Britains and the Count of the Saxon Shore. His troops were the main field army (comitatenses) in Britain and not the frontier guards (limitanei) commanded by the other two.
The first count in Britain was Gratian the Elder, the father of emperor Valentinian I. It seems to have been an appointment during some unrecorded crisis at the time. A permanent office was created later in the 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 List of Offices the count 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 presumably was extinguished with the Roman withdrawal from Britain by 410.