In late October 1515, the government authorities of Siena were busy preparing the city for the visit of Pope Leo X de’ Medici, expected in the middle of the following month. Palaces were requisitioned to house the pontiff and his entourage, while artists set to work to create the ephemeral architectural elements for a grand all’antica triumph through the streets. Under the supervision of Vannoccio di Paolo Biringucci, artists of the caliber of Domenico Beccafumi and Sodoma worked on arches and other accessories. On the eve of the visit, the pope altered his itinerary and bypassed the city. Tapping a series of unpublished documents, this article discusses the preparations for the visit and explores the reasons for its failure, which were ultimately political, as the de facto signore of Siena, Borghese Petrucci, failed to reach a diplomatic agreement with the Medici pope.