Belinus the Great was a legendary king of the Britons, as recounted by Geoffrey of Monmouth. He was the son of Dunvallo Molmutius and brother of Brennius. He was probably named after the ancient god Belenus.
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. According to the story, 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 led his great army to the Italian peninsula and threatened to invade Rome. Outside Rome the two consuls, Gabias and Porsenna, sued for peace and offered wealth, tribute, and hostages as a sign of their submission. Belinus and Brennius accepted and took their great army to Germany. Soon after this movement north, Rome broke the 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).