By far the most significant of the Christian thinkers to emerge in the late Middle Ages was the Italian scholar Thomas Aquinas, a member of the newly formed Dominican religious order. An order that valued the tradition of scholasticism, the Dominicans used reasoning and inference as a method of education, rather than simply teaching Christian dogma. In this spirit, Aquinas set about reconciling Christian theology with the rational arguments put forward by philosophers such as Plato and Aristotle. As a priest, his concerns were primarily theological, but since the Church was the dominant political power at the time, the distinction between theological and political was not as clear-cut as it is today. In arguing for an integration of the rational and the dogmatic, of philosophy and theology, Aquinas addressed the question of secular power versus divine authority, and the conflict between Church and state that was growing in many countries. He also used this method to examine ethical questions, such as when it might be justified to wage war.

In his moral philosophy, Aquinas explicitly examines political issues, stressing that reasoning is as important in political thinking as it is in theological argument. As a starting point, he took the works of Augustine of Hippo, who had successfully integrated into his Christian beliefs the classical Greek notion that the purpose of the state is to promote a good and virtuous life. Augustine argued that this was in harmony with divine law—which, if adhered to, will prevent injustice. For Aquinas, steeped in the works of Plato and Aristotle, justice was the prime political virtue that underpinned his entire political philosophy, and the notion of justice was the key element in governance. Just laws were the difference that distinguished good government from bad, bestowing upon it the legitimacy to rule. It was also justice that determined the morality of the actions of the state, a principle that can most clearly be seen in Aquinas’s theory of a “just war.”

Warfare for the protection of Christian values could be justified in Aquinas’s thinking, including the First Crusade of 1096–99, in which Jerusalem was captured and thousands massacred.

Using Augustine’s arguments as his starting point, Aquinas agreed that although Christianity preached pacifism for its adherents, it was sometimes necessary to fight in order to preserve or restore peace in the face of aggression. However, such a war should be defensive, not preemptive, and waged only when certain conditions could be met. He called these conditions the jus ad bellum or “right to war”—which were distinct from the jus in bello, the rules of just conduct in a war—and believed that they would ensure the justice of the war.

  Aquinas identified three distinct basic requirements for just war: rightful intention, authority of the sovereign, and a just cause. These principles have remained the basic criteria in just-war theory to the present day. The “rightful intention” for the Christian meant one thing only—the restoration of peace—but it is in the other two conditions that we can see a more secular approach. The “authority of the sovereign” implies that war can only be waged by an authority such as the state or its ruler, while the “just cause” limits its power to fighting a war only for the benefit of the people, rather than for personal gain or glory. For these criteria to be met, there must be a properly instituted government or ruler bound by laws that ensure the justice of its actions, and this in turn needs to be based on a theory of legitimate governance, taking into account the demands of both the Church and the state.

The Geneva Convention consists of four treaties signed between 1864 and 1949—broadly based on the concepts of just war—defining fair treatment of soldiers and civilians in wartime.

This recognition of the role of the state and its authority distinguished Aquinas’s political philosophy from other thinkers of the time. His emphasis on justice as an essential virtue, influenced by his study of Plato and Aristotle, led him to consider the place of law in society, and this interest in law formed the basis for his political thinking. Unsurprisingly, given the increasing plurality of society at the time, this involved an examination of the differences between divine and human laws, and by implication the laws of the Church and those of the state.

  As a Christian, Aquinas believed that the universe is ruled by an eternal divine law, and that humans—as the only rational creatures—have a unique relationship with it. Because of our ability to reason, we are subject to what he calls a “natural law,” which we have arrived at by examining human nature and inferring a moral code of behavior. Far from being a contradiction of God’s law, however, Aquinas explained this as our participation in the eternal law.

Reason, he argues, is a God-given ability that enables us to devise for ourselves the natural law, which is—in effect—the way in which the eternal law applies to human beings in accordance with our nature as a social animal. However, natural law, which is concerned with morality and virtue, is not to be confused with the human laws that govern our day-to-day affairs, which we have created to enable the smooth running of our social communities. These man-made laws are, like their creators, by their very nature fallible, so they can lead to injustice, and their authority can only be judged by comparison with natural law.

The laws that we create for ourselves and our societies must be based on natural law, which in itself is a reflection of the eternal law that guides the entire universe.

While Aquinas attributes natural law to our propensity for rational thought, the emergence of human laws is explained by another aspect of human nature—our need to form social communities. This idea is very much the same as proposed by Aristotle in Politics—which Aquinas had written a lengthy commentary on—that man is by nature a “political animal.” The urge to form social communities is something that defines us as humans distinct from other animals. Like Aristotle, Aquinas recognizes that humans naturally form family units, which in turn come together as villages, and ultimately form political societies, such as city-states or nation-states, providing an ordered social framework. Although he agreed in principle with Aristotle that such a state was the perfect community, his conception of it was not the same as the ancient Greek understanding, which was not compatible with the views of the Church in the 13th century.

  According to the Greek philosophers, the aim of such a society was to enable its citizens to lead a “good life” in accordance with virtue and reason. Aquinas’s interpretation was subtly different, bringing it in line with Christian theology and his own ideas of natural law. For him, the role of political society was to enable its citizens to develop their powers of reason, and through this, to acquire an understanding of moral sense—in other words, the natural law. They would then be able to live well, in accordance with natural law, and—as Christians—in accordance with divine law.

The Kellogg-Briand Pact, signed in 1928 by 15 countries, forbade its signatories from starting wars. This accorded with Aquinas’s principle that war should only be used to restore peace.

The question that followed was this: what form of government is best suited to ensuring the aspirations of this political society? Again, Aquinas takes his lead from Aristotle, classifying various types of regime by the number of rulers and, crucially, whether their rule is just or unjust. Rule by a single individual is known as monarchy when it is just, but tyranny when unjust; similarly, just rule by a few is known as aristocracy, but when unjust, as an oligarchy; and just rule by the people is called a republic or polity, as opposed to the unjust rule of a democracy.

What determines whether these forms of government are just or unjust are the laws through which order is brought to the state. Aquinas defined law as “an ordinance of reason, for the common good, promulgated by one who has the care of the community.” This definition sums up his criteria for just rule. The laws must be based on reason, rather than divine law imposed on the state by the Church in order that they satisfy our human need to deduce for ourselves the natural law.

Aquinas goes on to explain that purely human laws are also necessary for the maintenance of order in society. Natural law guides our decisions of right and wrong, and the moral code that determines what constitutes a crime or injustice, but it is human law that decides what would be a fitting punishment and how this should be enforced. These human laws are essential to ordered, civilized society, and provide deterrents and incentives to potential wrongdoers to act with respect for the common good—and eventually to “do willingly what hitherto they did from fear, and become virtuous.” The justice of human laws is judged by how well they measure up to natural law. If found to fall short, they should not be considered laws at all.

  The second part of Aquinas’s definition, however, is perhaps the deciding factor in judging the justice of a system of government. The laws imposed should be in the interests of the people as a whole, and not those of the ruler or rulers. Only with such laws can the state provide a framework in which its citizens can freely pursue their intellectual and moral development. However, the question still remains: who should rule? Aquinas, like Aristotle, believed that the majority did not have the reasoning power to fully appreciate the morality necessary for ruling, which implies that government should not be in the hands of the people, but a just individual, monarch, or aristocracy. But Aquinas also recognized the potential for these individuals to be corrupted, and argued instead for some form of mixed constitution. Surprisingly, in view of his notion of the state existing to promote life according to Christian principles, Aquinas does not dismiss the possibility of a legitimate non-Christian ruler. Although his rule would not be perfect, a pagan could rule justly and in accordance with human laws, allowing his citizens to develop their powers of reason and eventually come to deduce a moral code. Living then according to natural law, they would in time become a Christian society.

Aquinas’s view of the requirements of a just war—rightful intention, authority, and just cause—still hold true today, and motivate many involved in anti-war movements.

When viewed from our modern standpoint nearly 900 years later, it might appear that Aquinas was simply rediscovering and repeating Aristotle’s political theories. However, when considered fully in context against a background of medieval Christianity, his views are revealed as a radical change in political thinking that challenged the conventional power of the Roman Catholic Church. Despite this, thanks to his scholarship and devoutness, his ideas were soon accepted by the established Church and have remained the basis for a large part of Catholic political philosophy to the present day.

  In the criteria for a just war—rightful intention, authority of the sovereign, and just cause—we can see how these principles fit Aquinas’s more general ideas of political justice based on natural law, and the principle of reason rather than divine authority. As well as influencing much subsequent just-war theory, Aquinas’s notion of natural law was embraced by both theologians and experts on law. Over the centuries, the necessity of human law would become a key issue in the increasing conflict between Church and secular powers in Europe, as emerging nation-states asserted their independence from the papacy.

The United Nations was established in 1945 after World War II with the intention of maintaining international peace and promoting principles that Aquinas would have called natural law.