These three works all mention Arthur, either directly or by allusion, but it is the Historia that deals with him extensively and is the most important by far. Geoffrey's main purpose is to extol the glories of the Celtic Briton ancestors of the Welsh, of how they became mighty but were eventually conquered by the Saxons, the ancestors of the English, through treachery and their own moral decay. Initially expanding a Welsh legend that traces Briton origins to the wanderings of Brutus and his band of Trojans, Geoffrey then narrates a lengthy group of fictional reigns up to the time of the Roman conquest. Britain is once again independent after the break with Rome but the usurper Vortigern brings Saxons to the island as mercenaries. The Saxon numbers increase until they rise up and pillage the land. At this troubled time enters Merlin, a conglomerate figure composed from two or three earlier legends. Vortigern is deposed and after some time Arthur rises to power.
Arthur's reign makes up the climax of the Historia. First, he deals with the Saxon/barbarian menace. After achieving victory and marrying Guinevere, a "Golden Age" of peace and prosperity follows. At this time Arthur also founds his famous order of knighthood. He then goes on to conquer Gaul from the weakened Romans and holds his stately court at Caerleon. Tribute and territory demands from Rome provoke another war. Though victorious, Arthur is betrayed at home by the rebelling Mordred, his nephew who had been left in charge. Arthur is forced to return home and confront the traitor. In doing so, he is mortally wounded and borne away to the Isle of Avalon. The Saxons eventually overrun the Britons, who retreat into Wales. The Historia "ends" in 689 with the prophecies of Merlin fortelling of a return to power by the Celts.
Whenever his dates are checked, as in the Roman period, Geoffrey emerges clearly as a writer of fiction and cannot be relied upon for facts. Following medieval tradition, he fully modernizes Arthur's court to the 12th century. Later, however, from Caesar on he is using what passed for real history at the time and some of his source materials can be identified - the Historia Brittonum, Bede and Gildas in addition to Roman historians. For the most part he is creating and aggrandizing very little data but in his preface he claims to be translating from a much fuller source, one "ancient book in the British language" (maybe Welsh but probably Breton) bestowed upon him by Walter, archdeacon at Oxford. This claim remains dubious as no copy of this source is extant. But the tale of Arthur scribed by Geoffrey cannot be fully accounted for from the aforementioned sources hinting at some unknown text of some kind. There is a possible tie to the Continent from the resonance with 5th century events in Gaul. Traces of a similar source are found in the preface to the Breton Legend of St. Goeznovius.
Geoffrey followed with the poetic Vita Merlini in an attempt at the resolution of discrepancies arising from the Historia. After learning more of the northern bard Myrddin (the primary model for the sorcerer), he realized that his version did not corroborate this new knowledge. This work, though clever, does not succeed.
Though Geoffrey's writings were not the lone source of Arthurian romance, they lent the tale an air of authenticity, named the major players and placed their adventures within a recognizable framework. The Historia was, for the most part, accepted as fact by medieval chroniclers but eventually doubt superceded belief. Edmund Spenser uses much of it in The Faerie Queen and episodes in it have been the basis for King Lear and other plays.