Middle Ages to the Modern World: Grotius and the modern laws of war

While various customs and traditions of war-making have been there since the beginning of written history, the idea of “laws” that all sides should obey crystalised in the context of early modern European politics. The philosophical challenges posed by the Renaissance and the changing patterns of war due to the technological military revolution of the time brought forward a whole new set of ideas about how war should be conducted.
Religion-based customs of waging a war had already been discussed for centuries in the earlier theological interpretation of war making (e.g., St Augustine, St Thomas Aquinas), so it is unfair to say that the laws of war were something absolutely new and unheard of. Because of this we can assess the importance and effects of these changes and attempt to answer whether the transformation experienced in the Renaissance and post-Westphalian period was indeed as revolutionary as it seems.

To do we will examine the highlights of jus ad bellum and jus in bello as they were held during the Middle Ages and Renaissance until the Treaty of Westphalia.

Jus ad bellum refers to the justice of going to war, while jus in bello refers to the justice of acts within war; while the concepts of jus ad bellum and jus in bello are anachronistic (Kolb, 1997), we will use them for the sake of convenience without implying their use by the contemporaries.

First we will outline the medieval ideas about warfare following their development until the beginning of the military revolution and then examine whether there indeed were clear links between what Martin Roberts calls the “military revolution” and the birth of international laws of war. Finally we will contrast early modern ideas with those of the middle ages to find whether the change was indeed dramatic enough to call it a “revolution.”

In medieval Europe, the legal concept of war was a direct descendant of the post-Constantinian Christian dialogue between the pacifist religious dogma of the day and the need for justification of violence to answer real world challenges as had been dealt with under the Roman empire. Saint Augustine, writing in the late fourth century and who is often credited as the “just war” idea (Mattox, 2006 p.14), expressed the notion that the ability to wage war does not justify it and it can be used only in reaction: “For it is the wrongdoing of the opposing party which compels the wise man to wage just war (…)” (St. Augustine of Hippo, The City of God (Book XIX), Chapter VII.) This idea remained vaguely defined for over nine centuries until another man of cloth, Saint Thomas Aquinas, updated it and set it as a standard for medieval geopolitical scene. In his Summa Theologica he describes the requirements of jus ad bellum:

In order for a war to be just, three things are necessary. First, the authority of the sovereign by whose command the war is to be waged.(…) Secondly, a just cause is required, namely that those who are attacked, should be attacked because they deserve it on account of some fault. (…) Thirdly, it is necessary that the belligerents should have a rightful intention, so that they intend the advancement of good, or the avoidance of evil. (Thomas Aquinas, Summa Theologica ,II-II, Question 40)

What is important in Aquinas’ work is that not only he clearly defines who can initiate war and under what circumstances (leaving this right only to legitimate kings - princeps, which should in theory have finished all the conflicts between petty nobles and dukes), but he also adds a new requirement to start a war - an intention to finish it and establish a lasting peace (Reichberg, 2007).

Furthermore, in the same work he sets down the first clear instructions for jus in bello. For example, he argues that fighting on holy days is allowable when it is done in self-defence and that ambushes can be justified in the eyes of God as long as the war you are fighting is just (Thomas Aquinas, Summa Theologica ,II-II, Question 40). In contrast to modern conceptions of jus in bello, Aquinas does not offer any advice on who should or shouldn’t be targeted in war, which can imply that a just casus belli also justifies targeting the whole enemy population or at least that this aspect of war is of lesser importance than the avoidance of fighting on holy days.

Despite Aquinas’ omission on the status of combatants, it would be wrong to assume that in medieval Europe there was no prevailing distinction between combatants and non-combatants. This can be seen in the pax Dei (peace of God) clerical movement which since the 10th century had aimed at protecting clergymen and defenceless peasants, while stopping the plunder of Church and layman properties (Head, 1999). It is hard to judge the impact of this movement, but it can be considered to be one of the first organised attempts to implement protection for non-combatants that we can see influencing the development of the knights’ chivalric codes and later what we know as the laws of war.

An early international example of secular imposition of what could pass as the laws of war was the Holy Roman Emperor’s Golden Bull of 1356, in which the Holy Roman Emperor Charles IV declared that he prohibited his lords and vassals to wage wars that he did not consider to be just. It is also one of the first times that the devastation of the land is mentioned as a punishable war crime in the interstate context:

“We prohibit also each and every unjust war and feud, and all unjust burnings, spoliations and rapines, unlawful and unusual tolls and escorts, and the exactions usually extorted for such escorts(…) (The Golden Bull of the Emperor Charles IV 1356 A.D. Article 17)

However, since the Holy Roman Empire only had limited power over its nominal subjects, it is doubtful whether Charles’s Golden Bull really was adhered to. Furthermore, the decree applied only to wars within the Imperial jurisdiction, so these laws weren’t universal.
For the most part, restrictions on warfare in the Middle Ages were mostly vague and idealistic.

Jus ad bellum as proposed by Thomas Aquinas didn’t really have an impact on the number or intensity of conflicts: warring sides had to work harder to prepare their own political and ethical justifications of why their use of violence was legitimate.

Often that legitimacy had to be deeply rooted in the Christian ideas of divine judgment, so the Church and the Pope necessarily played a critical part in late medieval diplomatic wars. Despite this, the clerical ideas that emerged served more as guidelines rather than any kind of international laws in modern sense.

The Renaissance revived the intellectual debate. Scholars particulary from the University of Salamanca took up the leading role in this field. Francisco de Vitoria, another Dominican theologian in this field, made huge contributions to the foundations of what was to become international law. He was one of the main proponents of natural rights possessed by all people – being perhaps the first to enunciate a notion of human rights (Wiliams, 2004; Pagden, 1991, p.xvi). It is no surprise that he became a stout defender of the interests of Native Americans who at that time were coming under the yoke of the rapidly expanding Spanish Empire.

Some of his ideas about jus ad bellum were revolutionary for his time:

What may be a reason and cause of just war? It is particularly necessary to ask this in connection with the case of the Indian aborigines(…) my first proposition is: Difference of religion is not a cause of just war. ( Francisco de Vitoria, Relectiones Theologicae XII, Pt. 3, Argument 10)

The reason why this idea is so important is that for the first time a law of war was not dependent on the religion. All the earlier ideas about the customs of war in Europe were primarily inter-Christian, they weren’t international in the global sense. While wars that followed Reformation crushed Vitoria’s vision of banning religious casus belli, nonetheless this was a great step towards modernity. But that was not all – he also followed in Augustine’s footsteps declaring:

(…) only just cause for commencing a war, namely, a wrong received. ( ibid. Argument 13)

However unlike Augustine, Vitoria didn’t endorse warfare as a justifiable punishment (Patterson, 2007 p. 48). It is no surprise that his almost Kantian approach to jus ad bellum wasn’t implemented in practice, but for monarchs it was becoming ever more complicated to keep a good face and wage wars of aggression at the same time.

After the Reformation, the guiding role of religion was turned upside down: instead of providing a framework for common laws it became a dividing factor (Bennet, 2010). Realising this, Vitoria tried to redefine the now increasingly less and less influential chivalric and religious restrictions on allowable practices during wartime. He wrote that it is wrong to kill innocents even in a just war and mentioned the need to keep the violence in proportion to the injury received (Hensel, 2008 p. 15). He specifically states that:

(…) even in war with the Turks it is not allowable to kill children (…) because they are innocent. (…) same holds with regard to the women (…)(Ibid. Argument 36)

Vitoria and his peers from the School of Salamanca worked just before the beginning of Roberts’s military revolution. Relectiones Theologicae was published in 1557 (ibid. Introduction, Pt. IV, para. 3), creating an intellectual climate that was ready for ideas that were later proposed by the more recognisable name of Hugo Grotius and his contemporaries.

While Roberts himself doesn’t claim that the military revolution of the seventeenth century caused any significant change to international law (his theory is a lot more concerned with material side of war making), it is reasonable to assume that if such a major change indeed took place, it should have affected the legal side of war as well.

The first signs that this might indeed be the case were the attempts by jurists to balance the role of religious argument in debates surrounding the laws of war by attempting to rely more on moral or natural laws to support their ideas. However, while the Old Testament nonetheless remained among the most important sources for discussion of laws of war, the move to natural law can be clearly seen in the writings of Italian secular jurist Alberico Gentili in his De Iure Belli Libri Tres, where he suggests:

(…) war seems to be natural if you study nature. (…) it is natural that men should disagree; and the result is wars. (Alberico Gentili, De Iure Belli Libri Tres, Vol. 2 Ch. XII, p. 53)

Gentili’s work inspired the Dutch theologian and jurist Hugo Grotius, whose fame eventually outshined Gentili, Vitoria, Ayala and the rest of the scholars who greatly contributed to Grotius’ ideas.
Grotius lived during the heyday of the Thirty Years War, which was the most brutal and intensive conflict seen by Europe in a generation. The seventeenth century military revolution allowed a much more intensive war with greater casualties - probably around 8 million(Norman Davies,1997 p.568), and a slower resolution due to the fact that states now could regain their strength a lot faster. This in turn led to a highly controversial casus belli – differences among Christian denominations after the Reformation providing a putative justification for some.

Another fact that must have influenced Grotius was the introduction of Swedish Articles of War by Gustavus Adophus in 1621, just a few years before Grotius’ magnum opus „De jure belli ac pacis” was published. The Swedish Articles of War were a clear symptom of the military revolution: they were not ideological guidelines but were strict rules on how Swedish troops should behave both on and off the battlefield (Hagan, 2001). For example article 88 declared that abuse of women would be punished by death, while other articles also detailed under what circumstances soldiers were allowed to loot and/or burn. Protection for non-combatants became a lot more extensive than before - it wasn’t just about sparing their lives:

100. Neyther shall any tyrannise over any Churchman, or aged people, Men or Women, Maydes or Children, unless they first take Armes against them, under paine of punishment (…) (Articles of War decreed in 1621 by King Gustavus II Adolphus of Sweden, taken from Ögren, 1996)

Having in mind military methods of the era and the nature of warfare in general, it would be naive to assume that these laws put an end to atrocities. However, we can see that there was enough of an ideological and practical motivation for Gustavus Adolphus to encourage his brother-in-law William of Branderburg to execute his Swedish officers who broke the laws of war (ibid.) It is reasonable to assume that earlier military organisations have had their own codes of conduct, however the Swedish Articles of War were unique at the time in their bureaucratic detail and their harsh enforcement. We can see the influence clearly in Grotius’ work, where he tried to find the ultimate formula to rationally end conflicts.

For this reason, his work included both jus ad bellum instructions for kings and parliaments before they prepare to start a war and jus in bello for the soldiers who will be fighting on the ground. “The Rights of War and Peace” effectively cover all of the main questions that are relevant to the laws of war even in our times – Grotius speaks about rights of prisoners, protection for non-combatants, proportionality of force, restrictions on plundering of enemy territory and respecting neutral parties.

While all of these questions have already been raised earlier by various thinkers, Grotius greatly expanded them and suggested that both rights of nations and laws of war are supported by the natural law, which is a part of the divine law. What that meant in practice was that secular arguments were gaining as much importance and power as religious arguments. Grotius’s work strongly influenced the norms and practices of international conduct. (Another project of his that wasn’t directly linked to laws of war but nonetheless needs to be mentioned was the Mare Liberum: his work on the freedom of the seas, which became the foundation of maritime law). For example he asserts that:

It has long been a maxim, universally received among the powers of Christendom, that prisoners of war cannot be made slaves(…) (Grotius, Hugo (1625) The Rights of War and Peace)

He goes into detail on how prisoners shouldn’t be abused or mistreated and that their return should be arranged after the war. Here is an article from the Treaty of Westphalia of 1648 reflecting Grotian thought:

CX. Moreover, all Prisoners on the one side and the other, without any distinction of the Gown or the Sword, shall be releas'd after the manner it has been covenanted, or shall be agreed between the Generals of the Armys, with his Imperial Majesty's Approbation. (Treaty of Westphalia, Article CX)

While it can be argued that prisoners would have been released with or without Grotius’ input, usually the medieval (and Renaissance) practice of war was to ransom them out and in extreme cases prisoners could spend years behind enemy lines if there was no material motivation for their captor to release them.

The Treaty of Westphalia not only began the era of modern nation states, but it also played a huge role in creating the required conditions for the development of international law. Swedish Articles of War and Grotius’ work started a tradition that soon spread through Europe and its colonies. We can see this from the Royal Navy Articles of War of 1757, which imposed very strict behaviour on sailors or from the Articles of War of 1775 that were implemented by the American Continental Army. Both of these sources appeared more than a century later than Swedish Articles of War of 1621, but their layout and their main points remain almost the same. The Articles related to discipline, plundering and harassment of civilians are very similar in both Continental Army Articles of War of 1775 and Royal Navy Articles of War of 1757. The fact that these articles are similar despite representing different branches of service (army and navy) demonstrate to what extent Swedish articles of war influenced military thinking of the era.

Overall we can see that there was a dramatic change to the way laws of war were understood from Aquinas to Grotius. Medieval study of legitimacy of war was mostly confined to the causes and reasons for war, jus ad bellum. It’s hard to judge why the just in bello were ignored for so long. It’s possible that this was either because the primitive social-military structures weren’t able to enforce discipline among combatants or because of the fact that major thinkers of that time were men of the cloth who had little idea about how such norms could be implemented.

On the other hand, the Middle Ages gave us the main principle of the jus in bello – the Peace of God movement planted the idea of legitimate and illegitimate targets in the heads of medieval warrior aristocracy. Then Renaissance writings from the School of Salamanca scholars up to Grotius elaborated this idea, extending protections and restrictions further and further. When the military revolution exploded in Europe, we definitely see a change in the pace of development of the laws of war.

But the most likely explanation is that the devastation of the new wars renewed interest in finding the solution to the unending conflicts or at least to reduce their destructiveness to former levels. While Roberts himself didn’t emphasise the importance of this factor in his theory, we can see that the majority of the ground breaking works on this subject were published between late 1520s (first works by Francisco de Vitoria) and 1645 (Grotius’ death), after which no essential advancements were made in the field of international law until the 19th century. 1520 – 1645 period is not exactly the same as 1560 – 1660, however there is enough of an overlap and key events, such as Thirty Years war or publishing of the Swedish Articles of War to suggest that military revolution indeed triggered or at least to some extent vitalised the revolution of the international laws of war.

Dainius Tomas Balcytis
March 2014, Aberdeen

Primary sources:

• Alberico Gentili, De Iure Belli Libri Tres Vol. 2(John C. Rolfe trans. 1933 of the 1612 edition) Accessed through http://heinonline.org/ Retrieved on 25/3/14
• Francisco de Vitoria, Relectiones Theologicae XII, Pt. 3 The Second Relectio Of The Reverend Father, Brother Franciscus De Victoria, On The Indians, Or On The Law Of War Made By The Spaniards On The Barbarians (ed. ca.1917 by Scott, James Brown & ca.1964 by Nys, Ernest. Oceana Publications Inc., Wildy & Sons LTD. New York, London) http://en.wikisource.org/wiki/De_Indis_De_Jure_Belli/Part_3 Retrieved on 25/3/14
• Hugo Grotius, The Rights of War and Peace including the Law of Nature and of Nations, trans. by A.C. Campbell (NewYork: M. Walter Dunne, 1901) Accessed through Online Library of Liberty http://oll.libertyfund.org/titles/grotius-the-rights-of-war-and-peace-1901-ed Retrieved on 25/3/14
• Journals of the Continental Congress - Articles of War, June 30, 1775 Accessed through University of Yale Avalon project http://avalon.law.yale.edu/18th_century/contcong_06-30-75.asp Retrieved on 25/3/14
• Royal Navy Articles of War of 1757 Accessed through H.M.S. Richmond, Inc. http://www.hmsrichmond.org/rnarticles.htm Retrieved on 25/3/14
• St. Augustine of Hippo, The City of God Book XIX, Ch. VII. trans.by Marcus Dods
From Nicene and Post-Nicene Fathers (First Series, Vol. 2. Ed; Philip Schaff; Buffalo, NY: Christian Literature Publishing Co., 1887.) http://www.newadvent.org/fathers/120119.htm Retrieved on 24/3/14
• The Golden Bull of the Emperor Charles IV 1356, Article 17 taken from “Select Historical Documents of the Middle Ages” ed. Henderson, Ernest F. (London : George Bell and Sons, 1896)
• Thomas Aquinas, Summa Theologica ,Lit. translated by Fathers of the English Dominican Province ed. Kevin Knight( Second and Revised Edition, 1920) http://www.newadvent.org/summa/3040.htm Retrieved on 25/3/14
• Treaty of Westphalia: Peace Treaty between the Holy Roman Emperor and the King of France and their respective Allies , Accessed through University of Yale Avalon project http://avalon.law.yale.edu/17th_century/westphal.asp Retrieved on 25/3/14

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Article by Dainius Tomas Balcytis
Added Sat, 20 Sep 2014 20:36

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