The Sienese have entrusted the defense to Piero Strozzi, the son of Filippo Strozzi the Younger and Clarice de' Medici.
Athough Piero had in 1539 wed another Medici, Laudomia di Pierfrancesco, he is a fierce opponent of the main line of that family.
He had fought in the army led by his father and other Florentines exiled from France to oust the Medici from Florence, but, after their defeat at the Battle of Montemurlo, Piero had fled to France at the court of Catherine de' Medici.
In French service during the Italian War of 1542, and having raised an army of Italian mercenaries, he had been confronted by the Imperial forces at the Battle of Serravalle, where he had been defeated.
He had been in Scotland in 1548 supporting Mary of Guise of behalf of Henry II of France, where he had designed fortifications against the English at Leith and Haddington.
Named marshal of France in 1554, he now fights in the defense of the Republic of Siena against Cosimo de' Medici, leading French troops, as well as some Florentines who had been exiled by the Medici.
The Florentine troops approach Siena in the night of January 26, 1554.
After an initial failed assault, the Marquess of Marignano lays siege to the city, although his men are not numerous enough to totally cut it off from the countryside.
Both Baglioni and Montauto have failed to capture Pienza and Grosseto.
French ships harass the Florentine resupply lines at Piombino.
Cosimo replies to the initial setbacks by hiring Ascanio della Cornia with six thousand infantry and three hundred cavalry, and awaiting further Imperial reinforcements.
