If a republic is small, it is destroyed by a foreign force ; if it is large, it destroys itself by an inner vice.
This double vulnerability infects both democracies and aristocracies, be they good or bad. The problem lies in the thing itself ; there is no form that can provide a remedy.
Thus it is quite likely that men would ultimately have been obliged always to live under the government of one man alone, had they not thought up a sort of constitution which has all the inner advantages of the republican government and the outer strength of the monarchical one. I refer to the federative republic.
This form of government is a convention by which several political bodies agree to become citizens of a larger state which they wish to form. It is a society of societies, making a new one which can grow by new associates who have rallied to it.
It was these associations that enabled the body of Greece to flourish for so long. With them the Romans took on the entire world, and by them alone the world defended itself against them ; and when Rome had reached the pinnacle of her greatness, it was through associations beyond the Danube and the Rhine, associations they had made out of fright, that the Barbarians were able to resist her.
It is for this reason that Holland,  Germany, and the Swiss leagues are regarded in Europe as permanent republics.
Associations of cities were once more necessary than they are today. A powerless republic  risked greater dangers. Conquest made it lose not only its executive and legislative authority, as today, but also every kind of property known to men. 
This sort of republic, able to resist outside force, can maintain itself in its greatness without being corrupted from within ; the form of this society anticipates all its disadvantages.
A man who wanted to usurp power could never be equally accredited in all the confederated states. If he became too powerful in one, he would alarm all the others ; if he subjugated one portion, the portion that remained free could resist him with forces independent of those he had usurped, and crush him before he had fully established himself.
If some sedition occurs in one of the confederated members, the others can put it down. If there are abuses introduced somewhere, they are corrected by the healthy portions. This state can perish on one side without perishing on the other ; the confederation can be dissolved, and the confederates remain sovereign.
Made up of small republics, it enjoys the soundness of the interior government of each ; and with respect to the outside world, it has by dint of association all the advantages of large monarchies.