Maxima Caesariensis, short for Britannia Maxima Caesariensis (Latin for "Britain of Caesar Maximus"), was one of the provinces of the Diocese of "the Britains" created during the Diocletian Reforms at the end of the 3rd century. It was probably created after the defeat of the usurper Allectus by Constantius Chlorus in AD 296 and was mentioned in the c. 312 Verona List of the Roman provinces. Its position and capital remain uncertain although, on the basis of its governor's eventual consular rank, it is usually considered to have consisted of Augusta or Londinium (London) and southeastern England.
Following the Roman conquest of Britain, it was administered as a single province from Camulodunum (Colchester) and then Londinium (London) until the Severan Reforms following the revolt of its governor Clodius Albinus. These divided the territory into Upper and Lower Britain (Britannia Superior and Inferior), whose respective capitals were at Londinium and Eboracum (York). During the first phases of the Diocletian Reforms, Britain was under the control of the Allectus's Britannic Empire as part of the Carausian Revolt. At some point after the territory was retaken by Constantius Chlorus in AD 296, the Diocese of the Britains (with its vicar at Londinium) was formed and made a part of Prefecture of Gaul. The Britains were divided among three, four, or five provinces, which seem to have borne the names Prima, Secunda, Maxima Caesariensis, and (possibly) Flavia Caesariensis and Valentia.
The placement and capitals of these late British provinces are uncertain, although the Notitia Dignitatum lists the governor of Maxima (originally an equestrian praeses) as having been elevated to consular rank. Scholars usually associate this with the administration at Londinium, which was also the capital of the diocesan vicar.
Describing the metropolitan sees of the early British church established by SS Fagan and "Duvian", Gerald of Wales placed "Maximia" in Eboracum (York) and Londinium in Flavia, saying the former was named for the emperor Maximus. Modern scholars disregard this, although they are not certain whether it was named for the western senior emperor Valerius Maximian or the eastern junior emperor Galerius Maximian. Birley has argued that Maxima and Flavia originally consisted of a single province, which received the name Britannia Caesariensis as a mark of favour for support against the rebel Allectus in 296. Although Flavia is usually thought to have been formed from the old province of Lower Britain, Birley proposes that Upper Britain was divided in two (between Prima and Caesariensis) and then three (Prima, Maxima, and Flavia). This repeats Camden's earlier theory (relying on Sextus Rufus) that Maxima was formed first and Flavia followed sometime after.
- Frere, pp. 198-199.
- Polemius Silvius's 5th-century Nomina Omnium Provinciarum gives six provinces, but Roman administration over the Orcades (Orkneys) is generally discounted.
- Dornier, Ann (1982). "The Province of Valentia". Britannia 13: 253–260. doi:10.2307/526498.
- Valentia is generally treated as a later formation and placed variously beyond the Wall, around the Wall, and in Wales. It may, however, have simply been another name for the British diocese as a whole.
- Notitia Dignitatum.
- Verona List.
- Although note that the Notitia gave the same rank to the governor of the disputed province of Valentia.
- Giraldus Cambriensis [Gerald of Wales]. De Inuectionibus [On Invectives], Vol. II, Ch. I, in Y Cymmrodor: The Magazine of the Honourable Society of Cymmrodorion, Vol. XXX, pp. 130–1. George Simpson & Co. (Devizes), 1920. (Latin)
- Gerald of Wales. Translated by W.S. Davies as The Book of Invectives of Giraldus Cambrensis in Y Cymmrodor: The Magazine of the Honourable Society of Cymmrodorion, Vol. XXX, p. 16. George Simpson & Co. (Devizes), 1920.
- Both are problematic, since there is no known reason to have named a British province after the eastern caesar but Constantius Chlorus's senior partner was not a caesar but an augustus.
- Stillingfleet, Edward. Origines Britannicæ: or, the Antiquities of the British Churches with a Preface, concerning Some Pretended Antiquities Relating to Britain, in Vindication of the Bishop of St. Asaph, New Ed., pp. 77 ff. Wm. Straker (London), 1840.
- Frere, Sheppard (1967). Britannia: a history of Roman Britain. Cambridge: Harvard University Press.