When military conquest is involved in the unification of local small units into a larger societal grouping, there is a tendency for the leadership, whether vested in a powerful personality or a ruling group, to overextend themselves , using their success at one level as a springboard and indicator of success at the next level. This very normal human tendency to overreach has consequences when there has not been a period of consolidation to firm up the political, economic and psychological unity of the core group; namely, the development of a more complex society, with heterogeneous cultural, language and social groups does not have the cohesiveness required to stay together and eventually the larger imperial unit breaks down and dissolves.
Sri Aurobindo contrasts several instances where an internal unity was achieved by related clans or tribes, with instances where force was used to create the larger unity. “In Egypt and Judaea it was successfully found even in that ancient cycle of historical evolution; but in the latter instance certainly, in the former probably, the full result came only by the hard discipline of subjection to a foreign yoke. Where this discipline was lacking, where the nation-unity was in some sort achieved from within,– usually through the conquest of all the rest by one strong clan, city, regional unit such as Rome, Macedon, the mountain clans of Persia,– the new State, instead of waiting to base firmly its achievement and lay the foundations of the national unity deep and strong, proceeded at once to overshoot its immediate necessity and embark on a career of conquest. Before the psychological roots of the national unity had been driven deep, before the nation was firmly self-conscious, irresistibly possessed of its oneness and invincibly attached to it, the governing State impelled by the military impulsion which had carried it so far attempted immediately to form by the same means a larger empire-aggregate.”
A number of examples in history bear out the eventual weakness of this attempt: “Assyria, Macedon, Rome, Persia, later on Arabia followed all the same tendency and the same cycle. The great invasion of Europe and Western Asia by the Gaelic race and the subsequent disunion and decline of Gaul were probably due to the same phenomenon and proceeded from a still more immature and ill-formed unification than the Macedonian. All became the starting-point of great empire-movements before they had become the keystone of securely built national unities.”
Sri Aurobindo, The Ideal of Human Unity, Part One, Chapter 12, The Ancient Cycle of Prenational Empire-Building–The Modern Cycle of Nation-Building, pp. 94-95