When Pope Benedict, in the safe recesses of a German university, began an academic lecture by quoting an obscure medieval text on holy war, he can scarcely have expected the uproar that ensued.

After an amiable word of thanks to his old university colleagues, the pope referred to a set of dialogues, probably held in the winter of 1391, between the emperor of Byzantium, Manuel II Paleologus, and an ‘educated Persian’. Their subject was the truth of Christianity and Islam. The emperor, speaking with what the pope called ‘a startling brusqueness . . . that we find unacceptable’, raises a question about the relationship between religion and violence, arguing that violence is incompatible with the nature of God and the nature of the soul.

The pope does not associate himself with these comments – and in the published version of the lecture explicitly distances himself from them. But by then the damage had been done and a highly complex set of references and citations that set the emperor’s disparaging remarks about the prophet Muhammad in strong relief was enough to enrage Muslim opinion.