The development from nucleus to empire and from empire to civilization creates three sets of political problems: those arising from the administration of the nucleus; those arising out of contacts between the nucleus and the circumference, between the associates and dependencies and the nucleus, and those arising out of the determination of the associates and dependencies to sever their connections with the nucleus, win their independence, and take part in the unceasing efforts to establish new nuclei, win the unending power struggle and shift the power center. Relationship between nucleus and periphery are the normal outcome of the expansion of a nucleus into an empire. Each growing urban center reaches out for an extension of its territory; for the food and raw materials required by a growing population; for markets that can absorb the goods and services exported by the urban center to pay for its necessary imports of food and raw materials. Politically speaking, the essential problem is to maintain a relationship that will keep the imports coming in and keep the exports going out. Imports may take the form of plunder seized by the strong in contacts and conflicts with weaker neighbors; tribute paid by the weak to the strong at the insistence of the strong, or trade in which each side gains something. Empire building involves all three methods. In virtually all instances the nucleus is richer and stronger; the periphery is poorer and weaker. In virtually all instances these relative positions have been the outcome of military operations in which each party has tried to impose its will upon its rivals. In each case the spoils went to the victor, who forced defeated rivals to cede territory, to pay tribute, to give hostages or in some other fashion to agree upon a settlement that left the victor richer and stronger and the vanquished poorer and weaker. Politically speaking, the relation of nucleus to periphery was that of superior to inferior. Where the discrepancy was very great it resulted in a relation of master and vassal or even master and slave. An empire or a civilization, consisting of a wealth-power center and a periphery of associated and dependent territories and peoples, led to a living-standard differential in favor of the center. It also involved the establishment of a political apparatus strong enough to perpetuate the relationship by collecting tribute and taxes from the weak and depositing them in the treasure chests of the strong. The outcome was a civil bureaucracy backed by a military or police strong enough to defend and perpetuate an unpalatable superior-inferior position. Once established, both the civilian bureaucracy and the military apparatus tended to maintain themselves, to extend their privileges and strengthen their positions. Since controversial issues, domestic and foreign, are generally decided by force or the threat of force, the military became the strong right arm of authority.