From the Decline and Fall of the Roman Empire:
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The division of Europe into a number of independent states, connected, however, with each other by the general resemblance of religion, language, and manners, is productive of the most beneficial consequences to the liberty of mankind. A modern tyrant, who should find no resistance either in his own breast, or in his people, would soon experience a gentle restraint from the example of his equals, the dread of present censure, the advice of his allies, and the apprehension of his enemies. The object of his displeasure, escaping from the narrow limits of his dominions, would easily obtain, in a happier climate, a secure refuge, a new fortune adequate to his merit, the freedom of complaint, and perhaps the means of revenge. But the empire of the Romans filled the world, and when the empire fell into the hands of a single person, the world became a safe and dreary prison for his enemies. The slave of Imperial despotism, whether he was condemned to drag his gilded chain in Rome and the senate, or to wear out a life of exile on the barren rock of Seriphus, or the frozen bank of the Danube, expected his fate in silent despair. To resist was fatal, and it was impossible to fly. On every side he was encompassed with a vast extent of sea and land, which he could never hope to traverse without being discovered, seized, and restored to his irritated master. Beyond the frontiers, his anxious view could discover nothing, except the ocean, inhospitable deserts, hostile tribes of barbarians, of fierce manners and unknown language, or dependent kings, who would gladly purchase the emperor's protection by the sacrifice of an obnoxious fugitive. "Wherever you are," said Cicero to the exiled Marcellus, "remember that you are equally within the power of the conqueror."
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Edward Gibbon, The History of the Decline and Fall of the Roman Empire, Volume the First (1776) and Volume the Second (1781), David Womersley, ed. (New York: Penguin Books, 1994), 106-107 (chapter three).
One of the main pillars of justification for a system of independent states, acknowledging no common superior, but bound by common rules of international law, is that it promotes international freedom. The Snowden Affair illustrates that tendency wonderfully, as it presents a situation in which a regime hardly distinguished by respect for human rights should nevertheless form a check on a free government becoming much less free. The revelations for which Snowden is responsible, especially those showing relentless spying on friends and allies--everybody, it seems, everywhere and at any time--deserve severe condemnation. His revelation of them was a service to the country, though the state will hound him forever. His protection against the unjust proceedings of his own government is in the hands of an unjust but independent government, and in the circuitous workings of the system of independent states he gets some protection for his brave act.
The eighteenth century English and Scottish Enlightenment was preoccupied with Rome and saw with dread the grim consequences of the Roman conquest and of universal empire. Far better in protecting human freedom, in the view of Gibbon, Hume, and Robertson, was the system of independent states.