This word _imperium_ stood for three different kinds of power. First, the king was supreme in matters of religion, for he was responsible for the good relations between the human and divine inhabitants of the city, for “the peace of the gods” as it was called. If this peace, or covenant, were not kept up, it was believed that the State could not prosper—the very life of the State depended on it.
But now let us note a point of the utmost importance in the development of Roman public life. The king could not perform this duty entirely by himself; no single man could have the necessary knowledge of all the details of ancient religious custom. So he was assisted by a small board of skilled experts called _pontifices_, perhaps also by another board of _augurs_, skilled in the methods of discovering the divine will by omens. Thus the _imperium_ in religious matters, though still legally unlimited, was saved from becoming arbitrary and violating ancestral custom: the king is entrusted with power which he uses in accordance with the advice of sages.
Secondly, _imperium_ stood for the supreme judicial power, for the maintenance of peace between individual citizens. The king had an unlimited power not only in deciding disputes but in inflicting punishments, even that of death. But here again, though his power was absolute, it was not arbitrary. Custom governed the State even more than he did, and his work was to see that custom was obeyed. In order to make sure that this duty was rightly performed, he was provided with a council of elderly men (_senatores_), fathers of families, whose advice custom compelled him to ask, though it did not compel him to take it. Here, then, the exercise of discipline was combined with a sense of duty and obligation, as in the life of the family; the Senate of the State was the same in principle as the council of relations in the family.
Thirdly, _imperium_ stood for the absolute power of the commander in war: and here, as we might expect, custom seems hardly to have interfered with it. A Roman king in war was outside the custom of his own State, beyond the reach of the protection of his own deities, and under the influence of unknown ones. Both before starting on a campaign, and before entering the city on its return, the army had to undergo certain religious rites, which show how nervous even Romans were about leaving their own land and gods. Custom could not rule here, and the power of the general in the field remained throughout Roman history not only absolute but arbitrary. Doubtless he could, and often did, not only ask advice but take it, but he was never even morally obliged to do so: in this one department of State activity the wise judgment of the Romans left the _imperium_ practically unhampered.