Malory follows his initial tale with “The Tale of Arthur and Lucius,” which is derived from his most important English source, the alliterative Morte Arthure. This poem was composed anonymously at the end of the fourteenth century or the beginning of the fifteenth, and is unusual in being itself the product of the combination of multiple sources. Both Malory's tale and the alliterative poem are large-scale developments of the story of Arthur's war against the Roman Empire. Malory's version opens with the arrival of Roman ambassadors in Britain to rebuke Arthur for failing to pay tribute to the empire. Arthur holds a war council and then decides to press his own claim to be the Emperor of Rome. He appoints Baldwin of Britain and Constantine to rule jointly in his absence, and then he leads his army onto the continent to engage the Roman forces. On the way he fights the giant of Mont St Michel. After several battles, he defeats the Romans, punishes rebellious European vassals, is crowned Emperor of Rome, and returns to Britain in triumph.

The story of the Roman War originated in Geoffrey of Monmouth's Historia, and most of the chronicles that followed Geoffrey include this episode. This tradition, of which the alliterative poem is a part, makes the Roman War Arthur's final and greatest achievement and closely links it to his downfall, as his absence makes Mordred's rebellion possible. The romance tradition also contains versions of the Roman War.