Pazzi conspiracy
In his territorial aggrandizement of the Papal States, his niece's son Cardinal Raffaele Riario, for whom the Palazzo della Cancelleria was constructed, was suspected of colluding in the failed Pazzi conspiracy of 1478 to assassinate both Lorenzo de' Medici and his brother Giuliano and replace them in Florence with Sixtus IV's other nephew, Girolamo Riario. Francesco Salviati, Archbishop of Pisa and a main organizer of the plot, was hanged on the walls of the Florentine Palazzo della Signoria. Sixtus IV replied with an interdict and two years of war with Florence.