Many modern principles of law, including such basic and fundamental notions as the division between public and private law, are an inheritance from ancient Rome.  The Roman Empire provided its citizens not only with the military glory and material luxury of pax Romana, or with its famous and extensive network of roads, but also with the stability and certitude of its legal system.  There was, however, a unique contradiction at the heart of Roman law: because Romans conceived of their empire’s boundaries as the very perimeter of the world and of as themselves as the only true humans, their legal system did not allow, theoretically, for a conception of international law – although in practice Romans required one.  Romans may have coined the term “the law of nations,” but their conception of this law was, in fact, as an outgrowth of their own municipal law. As a result, the most important legacy of the Roman Empire for subsequent ideas about international law was a contradictory affinity for and aversion to universal empire.   

Rome originated as a small community of farmers and tradesmen in the center of the Italian peninsula on the Tiber River as early as the tenth century BCE.  Its legions went on to subdue first their neighbors, including the Etruscans, and then to conquer Sicily, Carthage, Macedonia, Syria, and Egypt.  Eventually the Mediterranean became a Roman lake, mare nostrum.  The Emperors who succeeded the Republic added Britain, areas of central and eastern Europe to the Danube, Arabia, and Mesopotamia to Rome’s imperium, the extent of its legal authority.  One theory about the end of the Roman Empire’s expansion was that it had reached the extent of what the Greeks had called “the inhabitable world.”

Roman law was first set down in the Twelve Tables, said to have been composed between 451 and 450 BCE.  Later jurists gathered and compiled further enactments into codes of law, culminating in Justinian’s codification in the sixth century CE.  Cicero, like many Romans, saw law as the supreme expression of human rationality: he wrote, “[h]owever we may define man, a single definition will apply to all…For those creatures who have received the gift of reason from Nature have also received right reason, and therefore they have also received the gift of law.”  Because of the vast extent of the Roman Empire, and because of this subsumption of law, rationality, and humanity – itself due to an assimilation of the legal conception of imperium and the Stoic belief in a single human race – Roman law therefore acquired a theoretically universal province.  As Anthony Pagden has written “[t]o be a Roman citizen” – to be able to claim civis Romanus sum – “meant to acquire a legal identity and a place in a system of understanding and controlling human behavior that was intended to extend to the entire planet.”

But did it?

Romans believed that their law was, to a certain extent, human law; that to be human was to be Roman and therefore subject to law.  As Caligula often misspoke, “the Roman people, or rather I say, the human race.”  However, Romans also recognized a distinction between ius civile, the common body of law applicable to all Roman citizens, and ius gentium, the law that applied to foreigners and Roman citizens alike and aimed to regulate interaction between the two.  This distinction can be confusing, because ius gentium is often described or translated as a “law of nations,” but Roman law, because of its theoretical universality, did not allow for a notionally higher-order law that applied equally to Romans and non-Romans, or to the Roman Empire itself, in the way most modern thinkers would imagine a law of nations.  It was just as impossible for Romans to conceive of this higher-order law as it was for them to imagine an independent political entity beyond their imperium.  Individual Romans were therefore surely aware of peoples beyond their borders, and they may well have had dealings with them, but they did not consider them as members of communities in either cultural or political or legal terms.  Thus, ius gentium, the body of legal principles that ordered the interactions between Roman citizens and non-Romans, was itself understood as another facet or manifestation of the omnipresent Roman legal system!  Over time, because of the pervasive power and control of the Roman Empire, ius gentium became for practical purposes what the second century jurist Gaius called “the law observed by all nations.”

This idea of a law observed by all would have profound and lasting impact on European and international legal thought, though not in the way Romans had originally understood it.  The direct effect of Roman law was more closely linked to its conception of and claim to universality. 

After Constantine the Great adopted Christianity as the Roman state religion in 380 CE, the Roman legal conception of imperium was, according to Lactanius, “made congruent with religion.”  Christianity was a particularly conducive religion to Roman modes of universalist thought given Jesus Christ’s and his follower’s claims, in contrast to the often exclusive strictures of the faiths of antiquity, to offer revelation and salvation for all of mankind.  Rome’s orbis terrarium therefore became the orbis Christianus in a clever translation by Pope Leo the Great in the fifth century.  Thus, after the (political) fall of Rome in 476, the Catholic Church inherited Roman claims to imperium.

Throughout the Middle Ages, the Church continued to stake universal claims, not only in matters spiritual but also in those political and legal.  Charlemagne, the first person to provide political unity in western Europe since the sack of Rome, was crowned Emperor of the Romans in 800 by the Pope Leo III – but it was not until the crowning of Otto I, who controlled what was then known as the Eastern Frankish portion of Charlemagne’s divided heritage, in 962 that the entity which came to be called the Holy Roman Empire developed with any continuity.  Intended to provide an explicit and secular sword to the church’s first religious one, the Holy Roman Empire would never actually achieve anything more than ceremonial precedence among the emerging states of western Europe. It nonetheless symbolized a latent desire for the political and legal unity of the Roman Empire.

Increasingly, with the rise of a number of centralized dynastic states at the end of the medieval period, whenever the threat of empire or hegemony appeared in Europe – whether in the form of the Emperor himself at the time of Charles V, or in a bold and overconfident king like Louis XIV – it tended to be opposed by a constellation of powers, and this opposition was developed as doctrine as the balance of power.  Over time, the Holy Roman Empire and what it represented therefore became more and more of an anachronism in the Westphalian system of secular, territorial, and technically equal sovereign states.  But, in a final irony, it was only formally abolished in 1806 when Napoleon Bonaparte finally succeeded, albeit briefly, in uniting Europe once again under a single imperial standard and code of laws.