The troubles in Syria have re-opened the debate about the legality of military intervention by the international community and have made me think again about what international law is and its history, especially (obviously) in the Middle Ages. As one BBC correspondent has summed up the current problem: ‘The words “international law” convey the sense of a set of established international rules and authorities agreed by all nations, and easily understood and applied by them. Sadly that is far from the case, and in practice, it is difficult, if not impossible, to get definitive rulings in international law involving military intervention. There is no international court on hand to give the legal go-ahead to intervene.’ (For the full story, see http://www.bbc.co.uk/news/uk-politics-23855428)
This may seem far removed from the Middle Ages. For instance, most historians agree that there were no such thing as ‘international law’ in the medieval period, particularly since there were no clearly defined nation states between which a law could be agreed and implemented. This is of course a simplification of what international law is; a combination of treaties (bilateral as well as unilateral), legal practice (or custom) and general principles of law (usually considered to be those that apply in all major legal systems). It is this combination of what international law is that perhaps give rise to the many interpretations of the ‘law’ and the difficulties of applying it, in modern times as well as in the Middle Ages.
For the medieval period, when historians write or talk about ‘international law’, they tend to look to canon law as something that was understood and applied across the medieval west – that is, canon law was one of those general principles of law. But what about treaties and legal practice? Those have attracted significantly less research by historians of the medieval period, especially the early and high medieval periods, and hence two-thirds of what we consider international law in the contemporary world is largely missing for the Middle Ages, and not because there are no treaties or legal practices but because historians have not really investigated. I would argue that this means that the history of international law in the medieval period is currently misunderstood because historians are looking to canon law because of its, supposed or otherwise, universality, although, as stated at the beginning, universality is not easily achieved or expected even in 2013.
For more about treaties and international law and the problems and possibilities of applying modern concepts to the study of the Middle Ages, my article ‘Law or treaty? Defining the edge of legal studies in the early and high medieval periods’ has just been published in Historical Research, vol. 86, no. 233 (August 2013) http://onlinelibrary.wiley.com/doi/10.1111/hisr.2013.86.issue-233/issuetoc