The region north of Baghdad was of different geographical formation to the southern plain, and therefore less suitable for the birth and growth of a great independent civilization. Assyria embraced a chalk plateau of the later Mesozoic period, with tertiary deposits, and had an extremely limited area suitable for agricultural pursuits. Its original inhabitants were nomadic pastoral and hunting tribes, and there appears to be little doubt that agriculture was introduced along the banks of the Tigris by colonists from Babylonia, who formed city States which owed allegiance to the kings of Sumer and Akkad. After the Hammurabi period Assyria rose into prominence as a predatory power, which depended for its stability upon those productive countries which it was able to conquer and hold in sway. It never had a numerous peasantry, and such as it had ultimately vanished, for the kings pursued the short-sighted policy of colonizing districts on the borders of their empire with their loyal subjects, and settling aliens in the heart of the homeland, where they were controlled by the military. In this manner they built up an artificial empire, which suffered at critical periods in its history because it lacked the great driving and sustaining force of a population welded together by immemorial native traditions and the love of country which is the essence of true patriotism. National sentiment was chiefly confined to the military aristocracy and the priests; the enslaved and uncultured masses of aliens were concerned mainly with their daily duties, and no doubt included communities, like the Israelites in captivity, who longed to return to their native lands. Assyria had to maintain a standing army, which grew from an alliance of brigands who first enslaved the native population, and ultimately extended their sway over neighbouring States. The successes of the army made Assyria powerful. Conquering kings accumulated rich booty by pillaging alien cities, and grew more and more wealthy as they were able to impose annual tribute on those States which came under their sway. They even regarded Babylonia with avaricious eyes. It was to achieve the conquest of the fertile and prosperous mother State that the early Assyrian emperors conducted military operations in the north-west and laid hands on Mesopotamia. There was no surer way of strangling it than by securing control of its trade routes. What the command of the sea is to Great Britain at the present day, the command of the caravan roads was to ancient Babylonia.