The King Arthur in Geoffrey of Monmouth's History of the Kings of Britain is a courageous, generous, and sweet-tempered monarch who attains the throne at age fifteen after the treacherous Saxons poison his father, Uther Pendragon. Monmouth says of Arthur that he was
a youth of such unparalleled courage and generosity, joined with that sweetness of temper and innate goodness, as gained him universal love.
Many soldiers flock to Arthur to serve under him, and he is so generous in rewarding them that he soon runs into financial difficulties. Therefore, he decides to wage war on the Saxons to regain the ancestral lands that belong to him by birthright and to increase the wealth of his kingdom so that he can "enrich his followers." He says,
Since these impious and detestable Saxons have disdained to keep faith with me, I, to keep faith with God, will endeavour to revenge the blood of my countrymen this day upon them. To arms, soldiers, to arms, and courageously fall upon the perfidious wretches, over whom we shall, with Christ assisting us, undoubtedly obtain the victory.
Unlike in later romances about Arthur, where his knights do the fighting, he is a valiant warrior who leads his troops into battle and excels as a fighter. He also shows that he has two sides to his character: the king who is gentle to his own people but bloodthirsty in fighting against the Saxon enemy. Monmouth notes that in one day, for example, Arthur kills more than 400 Saxons on the battlefield with his sword Caliburn (not Excalibur).
After consolidating his kingdom, Arthur shows himself to be a wise king who pardons his enemies and rebuilds cities such as York.
Monmouth's Arthur is an exemplary king who takes good care of his people. However, Monmouth's account is considered no more than pseudo-history.