But first it was necessary to become rich. Without a Treasury the army would melt away. How could Carausius grow rich? By seizing London and pillaging the City? But then he would make the whole island his enemy. There was a better way, a more secret way. He redoubled his vigilance over the coasts, but he did not attack the pirates till they were returning laden with their plunder. He then fell upon them and recaptured the whole. But he did not restore the spoils to their owners: he kept them, and in this way became very quickly wealthy. Presently the peculiar methods of the Admiral began to be talked about; people began to murmur; complaints were sent to Rome. Then Carausius learned that he was condemned to death. He was therefore forced to instant action. He proclaimed himself Emperor with Maximian and Diocletian, and he made an alliance with the Franks.
So long as he could rely on his troops, so long as he was victorious, he was safe; and for a long time there could be no opposition. Britain and the legions then in the island acknowledged him. He crossed over and made his head-quarters at Clausentum (Bitterne), near Southampton. He was certainly some time at London, where he had a mint; and he ruled the country undisturbed for some years.
We know nothing whatever about his rule, but there is probably very little to learn. He kept back the Picts and Scots; he kept back the pirates—that is clear from his coins, which speak of victory. We must remember that the reign of a usurper differed very little from that of a recognised Emperor. He preserved the same administration conducted by the same officers; it was only a change of name. Just as the government of France under the Republic is practically the same as that under the Empire, so the province of Britain under Maximian knew no change except a change of proprietor when it passed to Carausius.
But the end came. Carausius had to fight when he was challenged, or die.
The two Emperors appointed two Cæsars. To one, Constantius, was given the Empire of the West. He began his reign by attempting to reduce the usurper. With a large army he advanced north and invested Gesoriacum, where Carausius was lying; next, because the port was then, as it is now, small and narrow and impossible of entrance, except at high tide, he blocked it with stones and piles, so that the fleet could not enter to support him.