The arrival of the Emperor himself in Britain, and the vigorous preparations Severus at once made, caused great alarm among the hostile tribes, and they sent ambassadors to sue for peace. They had hitherto easily obtained it; but it was not Severus’s intention to depart from his purpose of total subjugation, and he dismissed the ambassadors without a decided answer, and without avowing his purpose, and proceeded with his preparations. When these had been completed, and a larger force collected than had ever yet been arrayed against them, Severus left his son Geta in the province, and taking his son Antoninus with him, he ‘passed the fortresses and rivers which guarded the frontier, and entered Caledonia.’ Severus had seen that the nature of the country had hitherto in the main prevented the Romans from penetrating far, or their conquests from being permanent in the north. The numerous natural bulwarks, the wide-spreading woods, and the extensive marshes, interposed almost insurmountable obstacles. What are now extensive plains, well-watered straths, and rich carses, must then have presented the appearance of a jungle or bush of oak, birch, or hazel; the higher ground rocky and barren, and the lower soft and marshy. If the native tribes were for a time subdued, and their strongholds taken, they could not be maintained in such a country by the Romans, and the natives speedily regained possession. The policy adopted by Severus was the true mode of overcoming such obstacles—to open up the country and render it passable for troops by clearing the jungles, forming roads in every direction, and throwing bridges over the rivers, so as to penetrate slowly with his troops and enable them to continue in possession of the districts as they occupied them in their advance through the country.