The construction of an empire doesn't necessarily begin as a rational action, but his consolidation is always fruit of deliberate choices.
The consolidation of the empire generates the appearance of an imperial culture, that it enters other things depends on the valorization of the militarism and the cult of a citizenship and of a city in the other ones (Roman Empire), the cult of the metropolis in the colonies (Portugal and England), the cult of the State and of a race inside and out of the national territory (Nazi Germany) or the cult of a flag and of a "way of life" inside and out of a State (USA).
Besides the depreciation of the militarism, the destruction of each empire type depends on a specific strategy. The Roman Empire was corroded slowly by the depreciation of the Roman citizenship. The Portuguese and English empires were demolished by the nationalism in the colonies, the Nazi empire finished for the military demonstration that the Aryan race was not superior to the races considered inferior and the American empire can only be destroyed with the combat to the "American way of life".
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