Key protagonists in these debates included Erasmus, Luther and Machiavelli. Today we might call them intellectuals, yet mostly they did not travel, and direct contact with the Ottoman Empire was scarce or nonexistent. Nor were they well disposed to its predecessor, the Byzantine Empire, whose fall presented them with an intellectual conundrum: how were they to explain the irresistible advance of the Ottomans across the Balkans and the inability of Christian Europe to hold the line? They also felt compelled to incorporate this significant new threat into their vision of a world order, to...