It chronicles the lives of the kings of the Britons over the course of two thousand years, beginning with the Trojans founding the British nation and continuing until the Anglo-Saxons assumed control of much of Britain around the 7th century.

When events described, such as Julius Caesar's invasions of Britain, can be corroborated from contemporary histories, Geoffrey's account can be seen to be wildly inaccurate.

He claims that he was given a source for this period by Archdeacon Walter of Oxford, who presented him with a "certain very ancient book written in the British language" from which he has translated his history.

Then follows a dedication to Robert, Earl of Gloucester and Waleran, Count of Meulan, whom he enjoins to use their knowledge and wisdom to improve his tale.

Brutus defeats the giants who are the only inhabitants of the island, and establishes his capital, Troia Nova ("New Troy"), on the banks of the Thames; later it is known as Trinovantum, and eventually renamed London.

When Brutus dies, his three sons, Locrinus, Kamber and Albanactus, divide the country between themselves; the three kingdoms are named Loegria, Kambria (North and West of the Severn to Humber) and Albany (Scotland).

Dunvallo's sons, Belinus and Brennius, fight a civil war before being reconciled by their mother, and proceed to sack Rome.

After his conquest of Gaul, Julius Caesar looks over the sea and resolves to order Britain to swear obedience and pay tribute to Rome.

After Guiderius is killed in battle with the Romans, his brother Arvirargus continues the defence, but eventually agrees to submit to Rome, and is given the hand of Claudius's daughter Genvissa in marriage.

When Octavius passes the crown to his son-in-law Maximianus, his nephew Conan Meriadoc is given rule of Brittany to compensate him for not succeeding.

The Britons are immediately besieged by attacks from Picts, Scots and Danes, especially as their numbers have been depleted due to Conan colonizing Brittany and Maximianus using British troops for his campaigns.

In desperation the Britons send letters to the general of the Roman forces, asking for help, but receive no reply (this passage borrows heavily from the corresponding section in Gildas' De Excidio et Conquestu Britanniae).

After Constantine's death, Vortigern assists his eldest son Constans in succeeding, before enabling their murder and coming to power.

Some of the prophecies act as an epitome of upcoming chapters of the Historia, while others are veiled allusions to historical people and events of the Norman world in the 11th–12th centuries.

After Aurelius Ambrosius defeats and kills Vortigern, becoming king, Britain remains in a state of war under him and his brother Uther.

When the king dies, a comet taking the form of a dragon's head (pendragon) appears in the night sky, which Merlin interprets as a sign that Ambrosius is dead and that Uther will be victorious and succeed him.

In the meantime, Arthur conquers most of northern Europe and ushers in a period of peace and prosperity that lasts until the Romans, led by Lucius Hiberius, demands that Britain once again pay tribute to Rome.

Arthur defeats Lucius in Gaul, intending to become Emperor, but in his absence, his nephew Mordred seduces and marries Guinevere and seizes the throne.

Arthur returns and kills Mordred at the Battle of Camlann, but, mortally wounded, he is carried off to the isle of Avalon, and hands the kingdom to his cousin Constantine, son of Cador and Duke of Cornwall.

Geoffrey claimed to have translated the Historia into Latin from "a very ancient book in the British tongue", given to him by Walter, Archdeacon of Oxford.

The medieval works are deemed to be drawn from traditional Celtic materials and are supposed to have deep roots to the point that the publications are considered reliable and not the imagination of one man, aka Geoffrey.

Then, it was merely a century later that Geoffrey's 'The History of Kings' was rewritten and translated in Cistercian monasteries at locations such as Whitland, St Davids Cathedral and later the Strata Florida Abbey in Wales, and then translated by Hywel Fychan ap Hywel Goch of Buellt into Latin-Welsh during 1250 in the manuscript Chronicle of Kings (Welsh: Brut y Brenhinedd).

Geoffrey's work was to do with the history of the British kings who hailed from the Greek Empire and specifically from after the Trojan War (fall of Troy).

Also, another king to cite the books was Prince Owain Glyndwr, he spoke of the Galfridian works Geoffrey to Robert III of Scotland.

[11][12][13] The work of Geoffrey of Monmouth was considered to take inspiration from the Latin publication of the life of 11th and 12th century King Gruffudd ap Cynan (written from 1137), finished by Gerald of Wales.

The history of Geoffrey forms the basis for much British lore and literature as well as being a rich source of material for Welsh bards.

The prophecies of Merlin in particular were often drawn on in later periods, for instance by both sides in the issue of English influence over Scotland under Edward I and his successors.

[citation needed] The variant title Historia regum Britanniae was introduced in the Middle Ages, and this became the most common form in the modern period.