"We use power, of course, in the international fields in a way which is the exact contrary to the way in which we use it within the state. In the international field, force is the instrument of the rival litigants, each attempting to impose his judgment upon the other. Within the state, force is the instrument of the community, the law, primarily used to prevent either of the litigants imposing by force his view upon the other. The normal purpose of police — to prevent the litigant taking the law into his own hands, being his own judge — is the precise contrary of the normal purpose in the past of armies and navies, which has been to enable the litigant to be his own judge of his own rights when in conflict about them with another."