The notion of legitimate self-defence, that persons or countries being violently and unjustly attacked may rightfully defend themselves by force, is ancient. The ongoing war of aggression against Ukraine has made the question most topical in the international law context. Through my own research, I can offer some facts of the history of this legal principle especially in a Nordic perspective.
In Antiquity, the right of self-defence became an incontrovertible cornerstone of natural law that applied universally to everyone regardless of religion or nationality. The Roman third-century jurist Ulpian, who is known for his basic legal maxims “to live honestly, not to injure another and to give everyone their due”, is also cited in the Digest coining the right to defend oneself when under attack. Considering it part of natural law, Roman law endorsed the principle that one had the right to repel force with force and armed attacks with the use of arms.
The Decretum of Gratian (fl. ca. 1120-1150) was perhaps the key work of the twelfth century when Roman law was “rediscovered” and canonical jurisprudence started to develop rapidly. Citing Isidore of Seville (ca. 560-636), Gratian observed that the right of legitimate self-defence (uiolentiae per uim repulsio) was one of the basic principles of natural law. Stephen of Tournai (1128-1203), one of Gratian’s commentators, added in his Summa (ca. 1160) that “all legislation and all law permitted one to repel force with force with moderation” for one’s self-preservation. These authors only represent some examples of the dozens of medieval learned men – jurists, theologians and philosophers – discussing the legitimate right of self-defence.
Medieval Scandinavian law did not use the same logic as learned law. Rather, as the laws only partially operated on individual guilt, they used a different expression: a killing could be “adjudicated uncompensated” (vgildan, ogildher, Sw. ogill).
Most slayings, even accidental ones, required compensation and/or punishment in the Scandinavian medieval legal cultures. Nevertheless, if a person was killed while committing an unlawful and objectionable act, the circumstances made the slaying acceptable so that no compensation, fines and/or other punishment were required from the perpetrator. Indeed, Swedish medieval law allowed a person to kill one’s aggressor with impunity for example in case of a fleeing thief, killing one’s rapist, killing an assailant during a breach of the peace of the home or during a robbery in the forest. However, the laws did not have a special word for self-defence.
While nödhvärn (verb nödhväria), self-defence, is known to have been used in Sweden in the late fifteenth century, it is probably a loan word from medieval Danish or possibly directly from the Middle High German nôtwęr. The learned Latin expression for self-defence (vim vi repellere) discussed above can be found in relatively few documents written in medieval Sweden. One such example is a Latin-language peacekeeping document from 1288 between King Magnus III Barn-lock (r. 1275–1290) of Sweden and the town of Visby.
The logic of the norms of self-defence may have worked differently in medieval Scandinavian and learned law, but both cases demonstrate the universality of the right of self-preservation by guiltlessly repelling unjust aggression with force. During the early modern period, the principle of self-defence in its “learned”, international form also became part of the Swedish legal system.
In fact, national legal systems across the world have adopted the principle of self-defence in criminal law, and as mentioned above, it has an important role in international law as well. As the Romans pointed out more than 1,800 years ago, justice and law authorize one to defend oneself against an unwarranted violent attack. This is what Ukraine is doing as I write. It is a natural right that cannot be denied of any person or any nation.
Professor of Legal History, University of Turku
 Institutiones 1,1,3: “honeste vivere, alterum non laedere, suum cuique tribuere.”
 Digesta 188.8.131.52: “Ulpianus 69 ad ed. Vim vi repellere licere cassius scribit idque ius natura comparatur: apparet autem, inquit, ex eo arma armis repellere licere.”
 Decretum Gratiani, D. 1 c. 7: “Quid sit ius naturale. [Isidor. eod. c. 4.] Ius naturale est commune omnium nationum, eo quod ubique instinctu naturae, non constitutione aliqua habetur, ut uiri et feminae coniunctio, liberorum successio et educatio, communis omnium possessio et omnium una libertas, acquisitio eorum, quae celo, terra marique capiuntur; item depositae rei uel commendatae pecuniae restitutio, uiolentiae per uim repulsio.”
 Stephen of Tournai, Die summa über das Decretum Gratiani. Edited by Johann Friedrich von Schulte. Giesen, 1891. Reprint Aalen: Scientia, 1965, Summa to D. 1 c. 7, p. 10: “Vis enim vi repellere omnes leges et omnia iura permittunt cum moderatione tamen inculpatae tutelae.”
 For a couple of examples, see e.g. 9-10 Af mandrapi [On Homicide], Äldre Västgötalagen (Older Law of West Gothia) in Corpus Iuris Sueo-Gotorum Antiqui, 1: Westgöta-Lagen. eds. H. S. Collin & C. J. Schlyter. Stockholm, 1827, Z. Haeggström, 14-15; 20-21 Dræpare bolkær [Chapter on Homicide], Yngre Västgötalagen (Younger Law of West Gothia), 127-128. See also K. F. Söderwall, Ordbok över svenska medeltids-språket, 2:1. Lund, Berlingska boktryckeriet, 1891-1900, 151.
 K. F. Söderwall, W. Åkerlund, K. G. Ljunggren, E. Wessén, Ordbok över svenska medeltids-språket, 4-5: Supplement, Lund, Berlingska boktryckeriet, 1925–1973, 567. For German, see Friedrich Kluge, An Etymological Dictionary of the German Language, trans. John Francis Davis. London. George Bell & Sons, 1891, 254.
 DS, 2, ed. Joh. Gust. Liljegren. P. A. Norstedt & Söner, Stockholm, 1837, doc. 970, 54: “Preterea promittimus quod si aliqua discordia casu sinistro inter nos & gotenens terre gotlandie exorta fuerit ipsam si bono modo poterimus sedabimus per nos & si hoc bono modo fieri non poteri-t antequam dominum nostrum regem super hac requisierimus ad vindictam aliquam contra eos non procedemus nisi ipsi in nos ita subito irruerint quod vim vi repellere compellamur.” https://sok.riksarkivet.se/sdhk?EndastDigitaliserat=false&Innehall=visby&TrycktUtgava=true&TrycktRegest=true&Texten=vim+vi+repellere&Brevtext=true&Extratext=true&Sigill=true&Original=true&MedeltidaAvskrifter=true&MedeltidaRegest=true&EftermedeltidaAvskrifter=true&EftermedeltidaRegest=true&AvanceradSok=False&page=1&postid=sdhk_1414&tab=post#tab