The difficulty has been that while it is quite easy for an ordered society to compel two individuals to settle their differences before a tribunal, in accordance with abstractly determined principles of law and reason, it is a vastly more difficult matter to compel two groups of individuals so to settle their differences. A large part of the history of all the great European countries has consisted in the progressive conquest and pacification of small but often bellicose states outside, and even inside, their own borders.[223] This is the case even within a community. Hobbes, writing in the midst of a civil war, went so far as to lay down that the "final cause" of a commonwealth is nothing else but the abolition of "that miserable condition of war which is necessarily consequent to the natural passions of men when there is no visible power to keep them in awe." Yet we see to-day that even within our highly civilized communities there is not always any adequately awful power to prevent employers and employed from engaging in what is little better than a civil war, nor even to bind them to accept the decision of an impartial tribunal they may have been persuaded to appeal to. The smallest state can compel its individual citizens to keep the peace; a large state can compel a small state to do so; but hitherto there has been no guarantee possible that large states, or even large compact groups within the state, should themselves keep the peace. They commit what injustice they please, for there is no visible power to keep them in awe. We have attained a condition in which a state is able to enforce a legal and peaceful attitude in its own individual citizens towards each other. The state is the guardian of its citizens' peace, but the old problem recurs: _Quis custodiet ipsos custodes?_ It is obvious that this difficulty increases as the size of states increases. To compel a small state to keep the peace by absorbing it if it fails to do so is always an easy and even tempting process to a neighbouring larger state. This process was once carried out on a complete scale, when practically the whole known world was brought under the sway of Rome. "War has ceased," Plutarch was able to declare in the days of the Roman Empire, and, though himself an enthusiastic Greek, he was unbounded in his admiration of the beneficence of the majestic _Pax Romana_, and never tempted by any narrow spirit of patriotism to desire the restoration of his own country's glories. But the Roman organization broke up, and no single state will ever be strong enough to restore it.