The Geneva conventions, which were signed in August 1949 in the aftermath of the horrors of the second world war, are generally accepted as the gold standard of international humanitarian law. They set the rules for the treatment of both combatants and civilians during wartime. But they were preceded by more than 12 centuries by a law drawn up by a monk amid the violence of seventh-century Ireland.

Adomnán, the Abbott of Iona, the great Columban island monastery off the western Scottish coast, was a man of many talents. Noted for his biography of St Columba, he was a scholar, teacher, mentor, poet, calendarist and negotiator. He was also known for his efforts to obtain legal guarantees from Pictish, British and Irish kings granting protection from violence to women, children, clerics and others, especially in wartime.

Conflict was commonplace in early medieval Britain and Ireland, we know that much from contemporary written sources, such as the Annals of Tigernach, Bede’s Ecclesiastical History of England and the Chronicon Scotorum. Civilian populations routinely ended up on the receiving end of military violence.

Adomnán knew only too well the effects of war trauma on non-combatants. He and his mother Ronnat had once encountered the horrific sight of a battlefield strewn with the mutilated corpses of mothers........

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