Roman theory of international relations was more primitive and elementary than that of the Greeks. Rome returned to the earlier idea of war as the natural relation among states. In her treaties of peace, however, she was not content with merely bringing war to an end, but established some permanent relationship of alliance with the former enemy.
At first Rome dealt with neighboring states on the basis of equality, but by clauses added to later treaties some form of vassalage was created in which the superior position of Rome was recognized. Foreigners were treated more liberally at Rome than in the Greek cities, and Roman foreign policy was guided by shrewder considerations of self-interest. In estimating the justice and legality of international acts, Rome always applied her own standards. A just war was one declared with due regard to Roman religious ceremonies and legal formalities.
The expansion of Rome into an empire was accomplished almost as much by diplomacy and statecraft as by force. Rome’s policy was to sow discord among different nations, to aid the weaker in overthrowing the stronger, and finally to bring both under her own control.
She carefully husbanded her own resources and used those of her allies whenever possible, and she frequently evaded treaties by subterfuge and practiced injustice under the guise of equity. Even after the establishment of her world-empire, Rome had to deal with her neighbors in war and peace. Embassies were received from India, Scythia, and from the kings of the Medes and the Iberians. Wars were waged with the Germans along the northern frontier.
However, Rome never treated these peoples as her equals. The Roman theory considered the empire as the only legal state, and from the point of view of international law other states did not exist. The jus gentium applied only to the peoples who were the allies of Rome. With others no legal relations whatever were recognized.
The establishment of the Roman Empire, while making international relations of any importance impossible by bringing practically the whole civilized world into one political system, nevertheless prepared the way for the later growth of international law.
The creation of a common citizenship, the maintenance of the Pax Romana, and the impartial administration of justice over many nations broke down the earlier isolation of states and the idea that foreigners were naturally inferiors and enemies. Men became accustomed to the idea of a common superior and a universal law, and these conceptions, especially prominent in the Middle Ages, were essential to the creation of a law among nations.
Equally important in this direction was the idea of a jus gentium, a body of rules and usages common to diverse peoples. In the minds of the later Roman jurists the general principles of the jus gentium were identified with the law of nature, and were thus considered to be universal principles applicable to all nations.
These ideas were appropriated by the founders of international Jaw toward the close of the Middle Ages, were applied in international practice, and gradually prepared the way for the modern conception of family of nations carrying on their relations according to definite legal principles.