In the 2nd century, Roman jurist Gaius defined ius gentium as what natural reason has established among all peoples. This definition separated the law of nations from civil law that applied only to Roman citizens. Cicero described this unwritten law as a higher moral obligation binding human beings beyond specific state requirements. A person exiled from Rome lost their citizen status but retained basic protections under this universal standard. Stoic philosophy influenced the view that these rules were innate in every human being. The jurist Ulpian later divided law into three branches: natural law governing animals and humans, civil law for specific people, and the distinctively human law of nations. Slavery existed under the law of nations even though natural law held all are born free. Hermogenianus, writing in the late 3rd century, listed wars, kingship, property boundaries, and commerce within this legal framework. These commercial contracts facilitated trade between different groups without requiring full citizenship.
War And Peace Treaties
The earliest form of international law was religious and pertained to the concept of the just war. Fetial priests performed ritualized declarations of war before any conflict could begin. Foreign ambassadors enjoyed protection under the law of nations because harming an envoy was considered a sacred violation. Livy recorded an envoy speaking about three kinds of treaties known as foedera. One type imposed terms upon conquered states where the victor decided what property to confiscate. Another allowed equally matched states to conclude peace on terms of equality through mutual agreement. The third existed when states never at war pledged mutual friendship without conditions. Local laws remained in force as long as they did not conflict with Roman authority. The praetor assigned to foreign affairs played a key role in extending civil law to these gentes. Questions arose over whether Rome was bound by agreements made by field commanders without Senate approval. No generally accepted principle of international law existed to resolve such controversies during the Republic era.Medieval Canon Law Synthesis
In late antiquity, Isidore of Seville enumerated principles of ius gentium focusing on peace treaties. He listed occupation, construction, fortification, wars, captivity, slavery, and treaties within this legal tradition. His work from around 600 AD included the inviolability of ambassadors and prohibitions against mixed marriages. By the Middle Ages, canon law contributed significantly to European legal traditions alongside Roman theory. Christianization of the Roman Empire brought new religious dimensions to existing legal concepts. Isidore noted that nearly every nation used these rules despite their diverse origins. The concept expanded to cover rights of regaining citizenship after captivity and the status of dediticii. These individuals became subjects of the empire but were excluded from universal citizenship under later constitutions. Medieval scholars integrated these ancient definitions into broader theological frameworks for understanding human relations.