The most significant contribution to Christian foreign policy is Thomas Aquinas’ development of the just war tradition (JWT). Thomas is typically associated with and praised for the strides that he made in the field of Christian theology, and this is certainly appropriate given his thorough writings and undeniable expertise on topics within this field. However, Christians owe much more to Thomas than merely his theological influence. When it comes to Christian foreign policy, no one has provided Christians with a firmer foundation or a more well-articulated stance than Thomas has. Here, I would like to specifically focus on Thomas’ development of the just war tradition and recognize the significance and influence that it has had throughout history.

Who Influenced Thomas

In order to rightly understand and appreciate Thomas’ work in this area, we must first—at least in brevity—acknowledge how JWT developed previously to Thomas’ contribution in his Summa Theologica. Well before Thomas, Augustine pioneered JWT in his De Civitate Dei (City of God) and other works, which earned him the title “father of Christian just war tradition.” This title is well deserved, as Augustine conveys many just war sentiments with a blatantly Christian conscience throughout this work. For instance, he explains the continuity between verses in scripture that praise peace (e.g., Matt. 5:9, which reads “blessed be the peacemakers”) with ideas of just war when he states that “peace should be the object of your desire… Therefore, even in waging war, cherish the spirit of a peacemaker, that, by conquering those whom you attack, you may lead them back to the advantages of peace.” In this exert and many like it, Augustine effectively makes the consistency between JWT and Christian teachings explicit.

Beyond merely tying JWT and Christian thought to one another, Augustine also gave the first criteria by which a war must be measured if it is to be qualified as just. Among these criteria are just cause (which is stipulated by the degree to which a war is necessary and reactive rather than desired and proactive), proper authority, and the propensity of a war (or action conducted in war) to result in peace and the common wellbeing. Augustine’s firm Christian commitment united with these formal conditions of constituting justice set the foundation for Thomas and his contribution to JWT.

Thomas’ Contribution to Just War Tradition

Although Thomas was not the creator of JWT, he is arguably the most well-known for it, and this recognition is largely due to the clarity and thorough treatment that he brought to the issue. Thomas developed JWT significantly by applying three criteria—much like Augustine—that aid in discerning what constitutes a just war. However, unlike Augustine, Thomas provides much needed clarity to these criteria, and he does so through the pages of a theology text and not a historical text, which establishes even more deeply the consistency between the Christian faith and JWT.

As noted above, Thomas’ stipulations for a war to be just are similar to Augustine’s, but they are narrower and more thoroughly explained. First, much like Augustine’s “proper authority” criterion, Thomas claims that in order for a war to be just, the ruler whose duty it is to defend the state must declare the war. Unlike his predecessor, Thomas leaves no ambiguity in this matter. That is, Thomas specifies that the head of state (emperor, president, king, etc.) must authorize the occurrence of war to be embarked upon, and this responsibility is on the head of state because it is his or her job as the protector and voice of the people to make such declarations. Thomas goes further in his explanation of this criterion of a just war and articulates that the responsibility of a head of state is to ensure the safety of his people and the common good of all people as far as it is possible for him to ensure these things.

The second criterion of a just war, according to Thomas, is that the cause must be just (e.g., self-defense, protection, prevention of a greater evil). This seems like an obvious requirement for any moral man (or state) to engage in conflict or war. But as history has shown us, not all men (or states) are moral, and not all wars are waged for just causes (e.g., the wars against the Native Americans, Hitler’s invasion of Poland, and the like). Therefore, a criterion like this is not only helpful for determining the justness of a war, it is paramount, and it has certainly been the benchmark for justifying wars throughout history (e.g., the allies’ response in World War II, the liberation of Kuwait, and the Israelites’ conquest of Canaan throughout the book of Joshua).

The final requisite that Thomas defends for the constitution of a just war is that the war must be declared and fought for good intention. In other words, in order for a war to be just, it must be fought for either the prevailing of good, the destruction of evil, or both. Ultimately, this portion of Thomas’ JWT differs from its other two components in that, unlike the previous criterion, this part is concerned with intention of entering into war rather than the logistics of entering into war. Another worthwhile distinction to make concerning this criterion is that both of these categories of “good intentions” result in the “common good” that was also mentioned under the first criterion of a just war. The common good was a consistent theme and motivation in Thomas’ political theory, so it is no surprise to see it appearing regularly throughout his JWT as well as the political philosophy and JWT of those who followed in Thomas’ footsteps.

Thomas’ Influence

Centuries after the death of Thomas, Francisco de Vitoria and Francisco Suarez continued the JWT on behalf of the Native Americans. The common belief in Europe in the sixteenth century was that, because the Native Americans were not Christian, it was acceptable to war against them and take their land as a duty of justice (or at least that was the common justification). In the early sixteenth century, Vitoria studied in Paris, where he was heavily influenced by Thomistic thought—likely because Thomas also studied and taught in Paris about 250 years earlier.

By 1510, reports had reached Europe that Native Americans were being mistreated, denied their natural rights and liberties, and were being stripped of their land by Europeans who desired it. These reports disturbed Vitoria, and he took action by means of lecturing on the subject and advocating on behalf of the Native Americans, which was highly unpopular at the time. It was these lectures and arguments that revived Thomistic thought in the sixteenth century.

Shortly following Vitoria’s efforts, Suarez attempted to combat European injustices against the Native Americans. Similar to Vitoria, Suarez used Thomistic arguments to show that the war against the Native Americans was in violation of a traditional Christian theory of just war. Because Europeans at that time were convinced that God desired for the Native Americans to be subject to them, Vitoria and Suarez were not successful in ending the war. But it is clear to us now that this philosophers’ disapprobation of the war displays the moral and theological superiority of JWT as opposed to its alternatives. This also shows the level at which Thomas’ JWT was venerated by true Christians and intellectuals centuries after his death.

The influence of Thomas in the realm of politics is little known, and unfortunately so. The link from his ideas to policies and governments is astounding and spans centuries. Richard Hooker adopted and modernized Thomas’ theory of natural law for his Laws of Ecclesiastical Polity. Following Hooker, John Locke yielded significant profit from Laws of Ecclesiastical Polity and extensively quoted it in his Second Treatise of Government. Finally, Locke’s influence stretches as far as to the founding of the United States. America’s Founding Fathers (particularly Thomas Jefferson) adored Locke’s Second Treatise of Government, and it not only laid the foundation for the US Constitution but is also quoted in the document.

More recently, the criteria that Thomas provided for discerning the justness of wars is still used as the benchmark for assessing wars today. World War II and the Persian Gulf War are the two most ubiquitously agreed upon examples of just wars in the modern world, and the measurements used to determine this are directly reliant on Thomas’. Thanks to the objective conditions that Thomas identified as being necessary for a war to be just, not only are we able to look back on past conflicts to determine their justness or unjustness, but we are also more astutely equipped to determine—as we did regarding WWII and The Persian Gulf War—the justness of a conflict before we enter into it.

In this way and many others, Thomas’ political influence (much like his theological influence) transcends borders, crosses oceans, and is timeless in its potency, application, and benefit. In fact, it can be argued that his political views have impacted the world to the present day and helped to create the greatest country that the world has ever known. In light of this, all are indebted to Thomas in some way, and in terms of Christian foreign policy, no one rivals Thomas’ contribution or prowess.

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Jimmy Lewis holds a BS in Biblical Studies and Theology as well as an MA in Philosophy of Religion from Liberty University. His scholarly interests are in ethics, religion, and foreign policy.

Photo Credit: Stained glass depiction of St. Thomas Aquinas speaking with Jesus, located in Saint Patrick Church in Columbus, Ohio. Via Wikimedia Commons.