In an effort to win the crown of Britain, Brennius and Belinus waged war between each other to determine who should succeed their father. Many battles were fought between the two brothers until a time came when their friends intervened and a compromise was decided upon. Belinus became the King of the Britons with Brennius as king of the north.
Five years later, Brennius wed the daughter of the King of Norway without consulting Belinus. Belinus invaded Northumberland and seized Brennius's land. The King of Denmark with Brennius's new wife landed in Britain by accident. Belinus imprisoned them and awaited the return of his brother. Brennius landed in Albany and demanded the return of all his lands and his wife. If not, he swore he would kill Belinus if they ever met in battle.
Belinus called to arms all of Britain against Brennius and the two armies met in the forests of Calaterium. The battle was fought ruthlessly and Belinus defeated the army of Brennius. Brennius fled to Gaul and Belinus became king over all the Britons. He emphasized the Molmutine Laws of his father and ruled justly.
Eventually, Brennius invaded Britain behind a massive Gallic army and met Belinus on the battlefield once again. Their mother, however, convinced Brennius to make peace, and the two brothers ruled their two realms in harmony with each other.
Following their unification, Belinus and Brennius merged their armies into one great one and invaded Gaul. After a year of warfare, the joint army managed to submit all the Frankish kingdoms in Gaul to their authority. Now with an even greater army, Belinus lead his great army to the Italian peninsula and threatened to invade Rome. Outside of Rome, the two consuls, Gabias and Porsenna, sued for peace and offered wealth, tribute, and hostages as a sign of their submittance. Belinus and Brennius accepted and took their great army to Germany. Soon after this movement north, Rome broke their treaty and marched north and Brennius went to fight the Romans while Belinus remained at war with the Germans (who were being helped by various other Italian troops).
After Brennius had left, the Italian troops who were reinforcing the Germans abandoned the Germans in a vain attempt to unite with the Roman soldiers on the other side of Belinus's army. Belinus learned of this and moved his army to a valley through which the Italians must pass. In the morning, Belinus attacked the Italians who were not in armour and unprepared for battle at that point in time. All day the Britons pursued the Italians until it was night.
Belinus decided to join forces with his brother who was sieging Rome. The Romans defended the city for many days and were successful in repelling the invaders. At last, Belinus decided to hang the hostages they were given in the treaty but it only enraged the Romans more. Finally, the two consuls put on armour and joined the men defending the city. They pushed the invaders back but Belinus was able to reform the lines and stop the attacks. Belinus continued forward until the walls were breached and the Britons invaded the city. Belinus left Brennius in Rome and returned to Britain.
He ruled in peace, building many new cities and restoring many decaying ones. Most important of the cities he founded was Kaerusc, which would be re-named Caerleon or the City of Legions when the Romans occupied Britain. This was the first reference to Caerleon-upon-Usk in Geoffrey's history. He continued using many of his father's laws and enacted a number of his own. Britain became more wealthy than ever before in this time.