The use of Tacitus as a source for political philosophy, however, began in this era, triggered by the Florentine Republic's struggle against the imperial ambitions of Giangaleazzo Visconti. Visconti's death from an illness did more than lift his siege of Florence; it sparked Leonardo Bruni to write his "Panegyric to the City of Florence" (c. 1403), in which he quoted Tacitus ("Histories", 1.1) to buttress his republican theory that monarchy was inimical to virtue, nobility, and (especially) genius. The inspiration was novel—Bruni had probably learned of Tacitus from Salutati. The thesis likewise: Tacitus himself had acknowledged that the good emperors Nerva and Trajan posed no threat to his endeavors.