The fundamental weakness of the commune or city state has always been its inability to protect itself from the aggressions of larger or more warlike neighbors, and its correlative inability to settle its own domestic differences without foreign interference.

On the other hand, when a state became sufficiently large and well organized to feel safe against alien aggression, it inevitably became the aggressor itself; and it inevitably carried the conquest of its neighbors just as far as it was able.

But domestic security, which is reached by constant foreign aggression, results inevitably in a huge unwieldy form of imperial political organization which is obliged by the logic of its situation to seek universal dominion.

The Romans made the great attempt to establish a dominion of this kind; and while their Empire could not endure, because their military organization destroyed in the end the very foundation of internal order, they bequeathed to civilization a political ideal and a legal code of inestimable subsequent value.
As long as men were obliged to choose between a communal or an imperial type of political organization,--which was equivalent merely to a choice between anarchy and despotism,--the problem of combining internal order with external security seemed insoluble.

They needed a form of association strong enough to defend their frontiers, but not sufficiently strong to attack their neighbors with any chance of continued success; and such a state could not exist unless its unity and integrity had some moral basis, and unless the aggressions of exceptionally efficient states were checked by some effective inter-state organization.