The young prince's cold-blooded massacre of his captives cast an enduring shadow upon his reign and dynasty. But it was henceforward plain to all that he was a man of stern resolve, who went straight to his end without scruples. He was regarded by many as the incarnation of Machiavelli's Prince. Guicciardini, who still pretended to act as mentor, withdrew from public life and when he died in 1540, it was immediately rumoured that the duke had caused him to be poisoned. This shows the estimation in which Cosimo was now held. He punished with death all who dared to resist his will. By 1540 sentence of death had been pro nounced against 43o contumacious fugitives, and during his reign 14o men and six women actually ascended the scaffold, without counting those who perished in foreign lands by the daggers of his assassins.
Cosimo bore a special grudge against the neighbouring republics of Siena and Lucca. Siena was an old and formidable foe to Florence, and had always given protection to the Florentine exiles. It was now very reluctantly submitting to the presence of a Spanish garrison, and, being stimulated by promises of prompt and efficacious assistance from France, rose in rebellion and expelled the Spaniards in 1552. Cosimo instantly wrote to the emperor, asking leave to attack Siena, and begged for troops. He then began negotiations with Henry II. of France, and, by thus arousing the imperial jealousy, obtained a contingent of German and Spanish infantry. Siena was besieged for fifteen months, and its inhabitants made a most heroic resistance, even women and children helping on the walls. But finally the Sienese were obliged to capitulate on honourable terms that were shame lessly violated.
In 1559 Cosimo also captured Montalcino, and thus formed the grand-duchy of Tuscany, but he continued to govern the new state--i.e. Siena and its territories—separately from the old. His rule was intelligent, skilful and despotic ; but his enormous expenses drove him to excessive exactions in money. Hence, not withstanding the genius of its founder, the grand-duchy held from the first the elements of its future decay. Cosimo preferred
to confer office upon men of humble origin in order to have pliable tools, but he also liked to be surrounded by a courtier aristocracy on the Spanish and French pattern. Cosimo was no Maecenas; nevertheless he restored the Pisan university, enlarged that of Siena, had the public records classified, and executed public works like the Santa Trinita bridge. During the great inundations of 1557 he turned his whole energy to the relief of the sufferers.
In 1539 he had married Eleonora of Toledo, daughter of the viceroy of Naples, by whom he had several children. Two died in 1562, and their mother died shortly afterwards. It was said that one of these boys, Don Garcia, had murdered the other, and then been killed by the enraged father. Indeed, Cosimo was fur ther accused of having put his own wife to death but neither rumour had any foundation. Worn by the cares of state and self indulgence, Cosimo, in 1564, resigned the government to his eldest son, who was to act as his lieutenant, since he wished to have power to resume the sceptre on any emergency. In 157o, by the advice of Pope Pius V., he married Camilla Martelli, a young lady of whom he had been long enamoured. In 1574 he died, at the age of 54 years and ten months, after a reign of 37 years, leav ing three sons and one daughter besides natural children. These sons were Francesco, his successor, who was already at the head of the government, Cardinal Ferdinand, and Piero.