If pressure from the outside world could be relied upon to bring about national unity, Mesopotamia would no doubt have become a single state on the lines laid down by the kings of Akkad. For the country was at all times exposed to great dangers. Civilized and prosperous, but lacking natural boundaries, it tempted mountaineers and steppe dwellers with the possibilities of easy loot. Raids could be dealt with by the cities, but the large-scale invasions, which recurred every few centuries, required a strong central government to be repelled. The safeguarding of the trade routes, too, went beyond the competence of individual cities, and one would expect them to have co-operated in a national effort. Indeed, we find an epic, “The King of Battle,” which describes how Sargon of Akkad, at the request of Mesopotamian merchants trading in Anatolia, went there with an army to champion their cause. The story may well reflect an actual occurrence, for Sargon’s grandson, Naramsin, built a strong castle at Brak on the Khabur, and the lumber used in its construction included not only poplar and plane, but also ash, elm, oak, and pine, which must have been imported.[129] The Akkadian kings thus undertook a task which occupied all succeeding rulers of the land. Even in the first millennium, the annual sweep of the Assyrian army up into the mountains of Armenia and down towards the west was a sustained and systematic attempt to keep the mountaineers in check; for, with the unlimited possibilities of retreat into their remote valleys, it was impossible to subject them permanently. From Sargon of Akkad on, kings knew that it was necessary to maintain a unified and centralized state; it was necessary to dominate the borderlands sufficiently to meet aggression there; in short, imperialism was the only guarantee of peace.