Different were the relations when a treaty of friendship existed. Persons and goods coming from one country into the other stood then under legal protection. So many foreigners came in the process of time to Rome that a whole system of law sprang up regarding these foreigners and their relations with Roman citizens, the so-called _jus gentium_ in contradistinction to the _jus civile_. And a special magistrate, the _praetor peregrinus_, was nominated for the administration of that law. Of such treaties with foreign nations there were three different kinds, namely, of _friendship_ (_amicitia_), of _hospitality_ (_hospitium_), or of _alliance_ (_foedus_). I do not propose to go into details about them. It suffices to remark that, although the treaties were concluded without any such provision, notice of termination could be given. Very often these treaties used to contain a provision according to which future controversies could be settled by arbitration of the so-called _recuperatores_.
Very precise legal rules existed as regards war and peace. Roman law considered war a legal institution. There were four different just reasons for war, namely: (1) Violation of the Roman dominion; (2) violation of ambassadors; (3) violation of treaties; (4) support given during war to an opponent by a hitherto friendly State. But even in such cases war was only justified if satisfaction was not given by the foreign State. Four _fetiales_ used to be sent as ambassadors to the foreign State from which satisfaction was asked. If such satisfaction was refused, war was formally declared by one of the _fetiales_ throwing a lance from the Roman frontier into the foreign land. For warfare itself no legal rules existed, but discretion only, and there are examples enough of great cruelty on the part of the Romans. Legal rules existed, however, for the end of war. War could be ended, first, through a treaty of peace, which was then always a treaty of friendship. War could, secondly, be ended by surrender (_deditio_). Such surrender spared the enemy their lives and property. War could, thirdly and lastly, be ended through conquest of the enemy's country (_occupatio_). It was in this case that the Romans could act according to discretion with the lives and the property of the enemy.