Guy Halsall's Worlds of Arthur: Facts and Fictions of the Dark Ages argues that King Arthur is one of the fictions of that period, "a total fabrication of the early ninth century or perhaps slightly earlier" (120). Those who see little justification for believing in Arthur's existence will welcome his arguments; those who think otherwise are not likely to change their views. However, the importance of this book lies not in its statements about the existence or non-existence of Arthur but in its reevaluation of the nature, extent, and time of the Anglo-Saxon invasions. Worlds of Arthur includes new interpretations not only of surviving texts, such as Gildas's On the Ruin and Conquest of Britain (De excidio et conquestu Britanniae) and the History of the Britons (Historia Brittonum) but also an extensive re-evaluation of the evidence provided by archaeological sites and burial grounds.
This is a clearly written, well-argued book intended for the general reader as well as the specialist. It avoids jargon, and its pages are uncluttered by footnotes. A weakness of the book, however, is that although Halsall includes a section that gives chapter-by-chapter guidance for further reading (309-320) and full references to the works mentioned in a bibliography (321-329), it is not always easy, as I indicate below, to see where he got his information.
Halsall divides the book into four parts, with each subdivided into chapters. Parts I to III "serve as a guide to why we do not know anything about Arthur and to why it is impossible to know whether he existed or not" (viii). Part IV sets out his own interpretation of the evidence, which he admits represents a personal view "not currently held by many people, and frequently controversial" (x). The controversial parts include the suggestion that traditional views of mass Anglo-Saxon invasions are questionable and that Gildas's discussion of a major war in his lifetime could refer to a Briton civil war and not to Saxon invasions. He believes that the Germanic tribes began moving into Britain in the 380s rather than after the withdrawal of the Roman legions in 420 and that they came in as mercenaries to offer support in Britain for the weakened Roman emperor Magnus Maximus, who was the inspiration for Vortigern, the Briton leader who, according to Historia Brittonum and Geoffrey of Monmouth, invited the Germanic tribes into Britain to fight against the Picts (191). He also believes that the migration of the Anglo-Saxons into Britain was not, as usually supposed, from east to west beginning with their arrival on the east coast. He considers it more likely that as invited mercenaries they lived in the center of the country where many Roman settlements were located. As Halsall admits, not all readers will agree with his interpretation of the evidence. Nevertheless scholars cannot easily dismiss them. This is an important study.
With support from work of David Dumville, Halsall demonstrates the lack of evidence for the Arthurian world portrayed in John Morris's Age of Arthur (1973) and dismisses Morris as "the Geoffrey of Monmouth de nos jours" (7-9). He later makes short work of theories about Arthur being Sarmatian, German, and Scottish (149-153) and his being identified with Riothamus (265-266). Whether Arthurian scholars agree with him or not, they should appreciate his survey of texts that tell of the historical Arthur and will need to consider his reinterpretations of Gildas's De excidio et conquestu Britanniae and Historia Brittonum, including his rather mind-numbing re-evaluation of the chronology of events in the latter (194-204). Halsall also argues, as Nora K. Chadwick did before him, that Historia Brittonum may have been influenced by ninth-century Welsh politics and written when it was "politically advantageous to compose a history in which the Britons united under a great, divinely favorued general" (172).  Halsall agrees with scholars who have argued that the account in Historia Brittonum of Arthur's twelve battles suggests deliberate attempts to provide a chiastic structure where elements are repeated in reverse order. He points out the importance of the number twelve in the passage: Arthur fights twelve battles in an account that contains 240 Latin words (20 x 12) and in a single day kills 960 men (80 x 12). He believes that the author may have known of only eight or nine battles which he converted to twelve. Oddly enough, he does not mention the numerological significance of twelve, perhaps because it had many meanings, but its signifying a perfect unit or something that has been completed might be relevant. Halsall admits that this does not mean that the account is wholly fictitious, simply that it is a literary composition, possibly based on some battles attributed to Arthur or possibly based on battles attributed to other warriors and combined here to make Arthur into a great hero (168-173).
This is an often persuasive book, but some statements are questionable.
Halsall believes that if Arthur had lived he would have ruled a small area and therefore "it is difficult to see how a warlord leading a realm no larger than a few modern parishes...could lie behind the legend of the great war-king" (120). He mentions elsewhere the existence of three minor rulers near the end of the sixth century who had the then-unusual name "Arthur" (24). Earlier scholars asked if they were given this name to honor a great predecessor. This commonly occurs today, just as one avoids names of leaders who have fallen into disrepute. On the one hand, in the United States the name "Franklin" was a popular first name because of Roosevelt; on the other, "Adolf" is no longer popular in Germany because of Hitler. Moreover, as evidenced by surviving Arthurian literature, legends about Arthur spread orally, and stories that circulate that way, whether folk tales or current gossip, tend to lengthen and become exaggerated. While some legends, like those of Brutus in Britain and Scota in Scotland, have no basis in reality, others such as those of Charlemagne, Richard the Lion Hearted, Mary Magdalene and possibly Robin Hood are exaggerated accounts of people who lived. Stories of Arthur could represent this type of legend.
Some comments in the book about the nature of medieval historical literature are questionable. Halsall writes "Unlike moderns, medieval people did not have a category of 'factual history' separate from what might today be thought of as 'historical fiction,' or even 'fantasy'" (51). This, in my opinion, is inaccurate. Admittedly, medieval chroniclers, like classical historians, felt free to embellish their accounts by inventing speeches or episodes that they thought might be appropriate for the occasion and also, like many modern historians, slanted their accounts to present a moral lesson or to persuade readers or listeners to take a particular point of view. However, medieval readers and listeners distinguished between "history" and "fiction," and chronicles were thought to be generally true accounts. They were not like the Bible, in which everything was true, nor like fables, in which everything was false, but occupied a middle ground in which most of the material was considered truthful. (Arthurian romances were usually distinct from chronicles, and on a truth-scale would fall between chronicle and fable.) The chronicler William of Malmesbury was one of the most highly regarded chroniclers, and his Gesta regum Anglorum (1125), written before Geoffrey of Monmouth's Historia regum Britanniae, refers to Arthur. Drawing presumably upon Historia Brittonum,  he points out that Arthur helped Aurelius, Vortigern's successor, overcome the barbarians, and adds "This is the Arthur about whom the trifles of Bretons [sic] now rave, one worthy not of being dreamed about in false fables [fallaces...fabulae] but proclaimed in true histories [veraces...historiae]."  William distinguishes here between "fables" which are fictive and "histories" which are true. Although Halsall does not mention this passage, he refers to William at one point, indicating that he had used a now-lost Latin poem for his account of the tenth-century King Æthelstan and "stylistic analysis of [William's] Latin...suggests that he might have been telling the truth" (141).
Elsewhere Halsall acknowledges that the late twelfth-century chronicler William of Newburgh complained about the fictive nature of Geoffrey of Monmouth's Historia. He points out, however, that "medieval people did not see the same distinction as their modern descendants do between legend and history. The point of 'history' was moral teaching, not facts about 'how it really was'" (6). He observes that William thought that Geoffrey was "making things up, but this was not crucial to a 'historical' compositions success or reputation...It's quite possible that William thought Geoffrey was lying because he didn't like the moral 'truth' he was proposing, not because he thought his sources were unreliable" (52). Admittedly William objected to Geoffrey's glorification of the Britons, whom Gildas had described as sinful, so yes, perhaps William did not like the moral that Geoffrey presented. However, William also says that Geoffrey's account was not factually true and was masquerading as history, and he tries to destroy the work's credibility. Contrasting Geoffrey with Gildas, he says that Geoffrey "wove a laughable web of fiction" about the Britons. He took the stories of Arthur "from the old fictitious accounts [fabulas] of the Britons, has added to them himself, and by embellishing them in the Latin tongue he has cloaked them with the honourable title of history." Geoffrey "doctored the events...with his own readily-added inventions...none except those ignorant of ancient histories can possibly doubt the extent of his wanton and shameless lying throughout the book." Geoffrey "gives space to fables without substance," presents "fictions about the achievements of the Britons," adopts "the false statements of others as authentic," and utters "lunacies...in praise of the Britons" that are "against the evidence of historical truth." He was motivated by "an uncontrolled passion for lying," and his account of Arthur's war on the Continent represents "undisciplined lying."  The later chronicler Ranulf Higden did not incorporate information from Geoffrey's account into his fourteenth-century Polychronicon because he could find no confirmation of Arthur's European conquests in chronicles written on the Continent. This too indicates concern with factual truth, not just with moral lessons.
Halsall discusses at various points in the book Leslie Alcock's archaeological work, particularly at South Cadbury, financed in part by "North American Arthurian fanatics" who thought it might have been Arthur's Camelot (39). Halsall lists several of Alcock's publications in his bibliography, but one that he omits and that, in fairness to Alcock, should have been mentioned is an article in which he wrote that his views had developed over the years and that his "position on the historicity of Arthur" had become "one of agnosticism." 
In the "Further Reading" section concerning his discussion of Riothamus (320), Halsall writes "The association of Riothamus with Arthur is found, inter alia, in [Jeremy] Du Quesnay Adams (1993)." This refers to a note that Adams wrote in support of Geoffrey Ashe, who was one of the first to make this association in a considerably longer article: "'A Certain Very Ancient Book: Traces of an Arthurian Source in Geoffrey of Monmouth's History," Speculum 56 (1981): 301-323 and again in his The Discovery of King Arthur (Garden City, NY: Anchor/ Doubleday, 1985), and again in The Arthurian Encyclopedia, gen. ed. N. J. Lacy (New York: Garland 1986) and its revision The New Arthurian Encyclopedia, gen. ed. N. J. Lacy (New York: Garland, 1991), both s.v. Riothamus. Anyone wanting to read a full argument that Riothamus was Arthur needs to read Ashe's work.
Halsall writes that Geoffrey of Monmouth's description of his source as the "story of the Britons from Brutus to Cadwallader" is one that "would fit the History of the Britons perfectly" (142). Although Historia Britonum was a source for Geoffrey, this gives a misleading impression of the nature of the work. Historia Brittonum, surviving in nine Latin recensions and a Middle Irish translation, would have offered Geoffrey only a brief outline of the material he included. In at least one version, which David Dumville has titled the "Vatican" recension, Cadwallader is not even mentioned, and while Geoffrey devotes about one third of his book to Arthur, the Arthurian material in Historia Brittonum is limited to the 240-word account of Arthur's twelve battles.  Geoffrey, like many medieval authors, was probably claiming a source that did not exist.
In his discussion of the languages of Scotland, Halsall correctly points out that Brythonic Celtic, the ancestor of Welsh, was spoken in western Scotland from early times. He questions modern Scots' efforts to promote Gaelic as the national language as opposed to Scots, which is a dialect of English: "It is not fanciful to suggest that the attempts to reject an Irish migration and to instate Q-Celtic Gaelic as always present in Scotland are linked with nationalist efforts to promote Gaelic-speaking as an official language, against 'Scots,' which is of course a dialect of English. The unpalatable fact (to nationalists) is that Scots was widely spoken in what is now Scotland from at least as early a date as Gaelic" (133). He gives no source for this other than an unpublished paper by a historian of early medieval Scotland whom he does not name. Admittedly, I am not familiar with attempts to reject an ancient Irish migration to Scotland or with the belief that Gaelic was always present in Scotland. The medieval Scottish origin legends indicate that the Scots settled first in Ireland and then moved into Scotland. Moreover, Halsall's statement that the Scots dialect of English was spoken from "at least as early a date as Gaelic" is incorrect. Linguists have long believed that Gaelic-speaking immigrants from Ireland moved into Scotland at least as early as the fifth century. In the mid sixth century, Irish missionaries migrated with St Columba to Iona and presumably their language would have been Gaelic. In the sixth century the Anglian kingdom of Bernicia extended into what is now southern Scotland, and people there would have spoken a dialect of Anglo-Saxon. Scots, however, was a much later Middle English dialect: it is derived primarily from the English of twelfth- and thirteenth-century immigrants into the Scottish lowlands from northern and central England. Scottish Gaelic was spoken throughout Scotland for much of the Middle Ages although since the fourteenth century it has been considered the language of the Highlands with Scots the language of the lowlands.  Scottish Gaelic is much older than Scots and thus has some claim to being the national language of Scotland.
Finally, a minor modification: Halsall mentions that in Malory's Morte Darthur Bedevere throws Arthur's sword into the lake "replacing Girflet, who played this role in earlier versions" (138). Malory's primary source for this was an English metrical romance, the Stanzaic Morte Arthur (ca. 1400), which assigned Bedevere this role, perhaps preferring a character drawn from Welsh tradition rather than French.
These objections, of course, concern minor points and do not diminish the book's overall importance, which lies in its re-examination of documentary and archaeological evidence and its plausible conclusions about the time and nature of Anglo-Saxon migrations into Britain. Scholars will have to respond to this book when they do further work on this topic.
1. Nora Chadwick, Studies in the Early British Church (Cambridge: Cambridge University Press, 1958).
2. William refers only to the battle of Mt Badon, at which, he says, Arthur went into battle with the image of the Virgin Mary on his armor, a detail similar to one in Historia Brittonum's reference to Arthur's eighth battle. William may have had an alternate source, or he may have been writing this from memory and forgot the exact details.
3. My translation from E. K. Chambers, Arthur of Britain (1927; rptd. Cambridge: Speculum Historiale, 1964), 250.
4. William of Newburgh, The History of English Affairs [Historia rerum Anglicarum], ed. & trans. P. G. Walsh & M. J. Kennedy, 2 vols. (Warminster: Aris & Phillips, 1988, 2007), vol. 1, pp. 28-37
5. Leslie Alcock, "Cadbury-Camelot: A Fifteen Year Perspective," Proceedings of the British Academy 68 (1982): 355-388, at 356.
6. David Dumville, ed., The Historia Brittonum: 3: The 'Vatican' Recension (Cambridge: D. S. Brewer, 1985).
7. See the Encyclopedia of the Languages of Europe, ed. Glanville Price (1998; rptd. Oxford: Blackwell, 2000), s.v. "Scots" and "Scottish Gaelic."