"The apparently impossible feat of uniting Sienese." Mendoza's fortress accomplished what centuries of Italian politics could not: it gave Siena's warring factions a common enemy.

Giorgio Vasari, Cosimo studies the taking of Siena (1563-1565). Palazzo Vecchio, Florence. Google Art Project.

By the early 1550s, the Italian Wars had entered their final phase. Charles V's empire stretched across most of the peninsula, enforced through garrisons, ambassadors, and client states. Siena was a Tuscan city-state wedged between Florence and the Papal States. It was one of the most internally divided cities in Italy. Its political life was organized around factional groupings called the Monti, which had been competing, allying, and betraying each other for centuries.

Don Diego de Mendoza, Charles's ambassador, was assigned to supervise Siena and decided the city needed a fortress. The Sienese emphatically disagreed. Mendoza told Charles the Sienese were requesting one. He told the Sienese that Charles had ordered it. He funded the construction out of his own pocket when neither party would pay.

Getting the Sienese to agree on anything had defeated better politicians than Mendoza. He managed it by being worse than any of them.

The French would claim the credit for the expulsion of the Spanish from Siena. Their support was timely, but discontent in Siena had reached the pitch where an uprising would probably have happened soon in any case. Mendoza had become fixated on the project of building a fortress there. He told Charles that it was the Sienese who were asking for a fortress, and the Sienese that it was Charles. The emperor agreed the plans Mendoza sent to him, and brushed aside the protests of the Sienese. Despite getting little money from Charles for the project, and very little money, materials or labour from the Sienese, Mendoza pressed on, paying for the work himself. He accomplished the apparently impossible feat of uniting Sienese of all factions and social classes behind one aim—but that aim was freeing themselves of Mendoza, the troops and the fortress. Given the attitude Charles was adopting, that meant throwing off their subordination to him, and that led to seeking the protection of the French.

Plans were laid for an uprising in Siena to coincide with incursions into Sienese territory by French troops from the neighbouring estates of barons sympathetic to the French, such as Nicola Orsini, conte di Pitigliano. False reports were deliberately circulated which deceived Mendoza into believing that it had been decided to attack Naples, so that he sent half the garrison to defend the Sienese ports. One of the conspirators planning the uprising, Amerigo Amerighi, was a member of the main executive body in the Sienese government, the Balia, and he contrived to get orders from the Balia to raise the militia in the territory, supposedly to face the threat of the Turkish fleet. Even most of the captains had no notion of the real reason why the militia was being mustered, before they marched on Siena.

When the militia appeared beneath the walls on 27 July, the garrison (Mendoza was in Rome) were unsure what to do. One gate was taken by force, the others opened to the militia. On the following afternoon, French contingents began to arrive, as the Spanish troops, together with 400 Florentine militia who had been sent in their support, were concentrating within the curtain wall of the fortress, all that had been built. Cosimo prepared to send more troops but held them back on receiving a message from the French that to oppose the uprising would be to oppose them too. Unwilling to enter into a war against the French, Cosimo began negotiating with the Sienese.

Mendoza had to order his men to conform to whatever terms were agreed. These were concluded on 3 August. The troops were to leave the fortress; the Sienese could demolish it, and should send all other foreign troops away after the Spanish had cleared the state (in fact a Spanish garrison would stay in Orbetello). They were to remain loyal to the Empire—but the Sienese were careful not to pledge continued loyalty to Charles. On 5 August, the troops left the fortress, and the Sienese began to tear it down with a will. Soon, the French were assuming greater powers over Siena than the Sienese wished them to have; Cardinal Ippolito d'Este, who came on 1 November as Henry's lieutenant-general there, was inclined to behave as though he were governor.

Christine Shaw & Michael Mallett, The Italian Wars 1494-1559 (Pearson, 2012), pp. 312-314.

Christine Shaw and Michael Mallett's The Italian Wars 1494-1559 (2012) covers sixty-five years of military and diplomatic chaos on the Italian peninsula, from the French invasion that shattered the peninsula's political order to the peace settlement that handed most of it to Spain. Shaw, a specialist in Renaissance Italian politics at the University of Warwick, and Mallett, who spent his career studying Italian military institutions, produced one of the most detailed English-language accounts of the period's military operations.

James C. Scott, Hannah Arendt, and other historians describe a recurring pattern in authoritarian governance: the state project that generates its own opposition. Mendoza's fortress is the physical case study. His fixation on the construction was so complete, and so deaf to local politics, that it created the one condition under which Sienese unity was possible: a shared enemy. The factions did not reconcile. They found something they hated more than each other.

The conspiracy was basically a heist through diplomatic paperwork. Amerigo Amerighi, a member of the Balia, used his position to issue official orders mustering the territorial militia against a Turkish naval threat that did not exist. False intelligence about a planned attack on Naples tricked Mendoza into sending half the garrison south. The gates opened, the French arrived, and the Sienese tore the fortress down: the structure Mendoza had paid for with his own money, the one he had lied to both his emperor and his hosts to build, dismantled by the people it was supposed to control.