On the night of 7 March 1623, the Prince of Wales and the Duke of Buckingham knocked on the door of the British embassy in Madrid. Their unsolicited arrival began one of the most bizarre episodes in British history, as the Protestant heir to the Stuart throne struggled to win the Spanish Infanta as his bride. The prince's visit marked the end of a decade of high-level negotiation to secure a marriage between the leading Protestant and Catholic royal families and heal Europe's century-old division into warring Christian camps. The effort was a diplomatic disaster. It split political and...