For the ancient Greeks it was clear. The ideal polis, a self-governing community, had a maximum of citizens. They disagreed on the actual number, but anything beyond a few ten thousand was considered to be incompatible with self-governance.
The argument was simple. A large group of people is unable to rule itself effectively. Imagine a meeting with a million persons, it will be clear that only a few can speak on such event. Consequently, self-governance – the ideal of classical republicanism – is only possible if the community is not too large.
Even untill the 18th century, the general idea was that republics had to be small states. Montesquieu wrote that republican government is most suited for small states, while monarchical government was most suited for medium-sized countries.
The problem with republics, as seen historically, was that they were vulnerable for aggression from larger states. For instance the ancient Greek city-states were in permanent fear of conquest by Persia or Macedonia, and later Rome.
Already in antiquity it was known that the most efficient way to govern a large and populous territory was by a king supported by a professional elite. It is clear that imperialism and self-governance are incompatible.
The question which classical republicans have to answer, is whether there is some way for self-governing communities to maintain themselves while being surrounded by kingdoms and empires?
Both Montesquieu and David Hume proposed federation as the solution for this problem, based on the success of the Dutch Republic – a confederation of several smaller states – and to some extent that of Switzerland.
The main purpose of the federal government is, according to Montesqiue and Hume, to provide external security to its constituent states. Domestic policy was the left as much as possible to the (city-)states. From this perspective it is clear that so-called “unitary republics” are incompatible with classical republicanism.
Centralization of power not only deprives citizens from the ability to active participation in politics – as opposed to mere voting – but also promotes political corruption and the abuse of power. Even more the centralization of power is one of the causes of growing citizens’ discontent with politics as they lose control of their own lives.
It is needless to say there are many issues that affect not only local communities and cities. Nevertheless issues should be determined at the level closest to the citizens, the principle of subsidiarity. Only if necessary policy should be made at the national level. And even if policy is set at the national level, the federal government should restrict itself to a broad framework and leave the details to the states and local communities.
Of course, it will be an open question what issues should be decided on what level. But every time power is upsurped by a higher level of government, the question should be whether it is possible to settle the issue at a lower level.