15. =The Roman Empire.=--The Roman state was now a monarchy. The Emperor was the head of the army, as well as the head of the state. Though he was often a cruel oppressor of the wealthy personages who lived in Rome itself, and whose rivalry he feared, he, for the most part, sought to establish his power by giving justice to the provinces which had once been conquered by Rome, but were now admitted to share in the advantages of good government which the Empire had to give. One consequence of the conquest of nations by Rome was that there was now an end to cruel wars between hostile tribes. An army was stationed on the frontier of the Empire to defend it against barbarian attacks. In the interior the Roman peace, as it was called, prevailed, and there was hardly any need of soldiers to keep order and to maintain obedience. 16. =The Invasion of Aulus Plautius.= A.D. =43=.--One question which each Emperor had to ask himself was whether he would attempt to enlarge the limits of the Empire or not. For a time each Emperor had resolved to be content with the frontier which Caesar had left. There had consequently for many years been no thought of again invading Britain. At last the Emperor Claudius reversed this policy. There is reason to suppose that some of the British chiefs had made an attack upon the coasts of Gaul. However this may have been, Claudius in =43= sent Aulus Plautius against Togidumnus and Caratacus, the sons of Cunobelin, who were now ruling in their father's stead. Where one tribe has gained supremacy over others, it is always easy for a civilised power to gain allies amongst the tribes which have been subdued. Caesar had overpowered Cassivelaunus by enlisting on his side the revolted Trinobantes, and Aulus Plautius now enlisted on his side the Regni, who dwelt in the present Sussex, and the Iceni, who dwelt in the present Norfolk and Suffolk. With their aid, Aulus Plautius, at the head of 40,000 men, defeated the sons of Cunobelin. Togidumnus was slain, and Caratacus driven into exile. The Romans then took possession of their lands, and, stepping into their place, established over the tribes chieftains who were now dependent on the Emperor instead of on Togidumnus and Caratacus. Claudius himself came for a brief visit to receive the congratulations of the army on the victory which his lieutenant had won. Aulus Plautius remained in Britain till =47=. Before he left it the whole of the country to the south of a line drawn from the Wash to some point on the Severn had been subjugated. The mines of the Mendips and of the western peninsula were too tempting to be left unconquered, and it is probably their attraction which explains the extension of Roman power at so early a date over the hilly country in the west.