The battle of Marciano (2 August 1554) saw the active field army of Siena suffer a heavy defeat, allowing their Florentine enemies to focus all of their efforts on the siege of Siena.
Siena had expelled its Spanish garrison in the summer of 1552. Despite an agreement not to let any foreign troops into the city, the Sienese let in a French garrison and began to improve their defences. An early Imperial attempt to retake the city, using troops from Naples, failed at the start of 1553, and the city was given another year of peace.
Towards the end of 1554 Cosimo de Medici, rule of Florence, decided that Siena posed a threat to his rule, partly because of the efforts of the Florentine exile Pietro Strozzi, who was attempting to encourage a revolt against him. In November Cosimo agreed to attack Siena in a secret treaty with the Emperor Charles V.
Strozzi reached Siena in January 1554, just ahead of the Florentines. He then left the city to inspect the defences of the Republic, and during his absence the Florentines managed to capture one of the city gates. They were unable to hold onto their gains, and a long siege began.
Strozzi proved to be an expert at mobile warfare. His opponent, Gian Medecino, Marquis of Merignano, was a successful artillery general, but at first he was unable to cope with Strozzi's mobility. Strozzi had successful appealed to Henry II of France, who decided to send reinforcements to help the defenders and Blaise de Monluc to rule the city. Strozzi broke through the siege lines, advanced north, crossed the Arno just to the west of Florence, and joined up with the French reinforcements. He then eluded an Imperial trap near Pisa and moved south to gather more reinforcements at Piombino. He then moved east, before bringing 17,000 men into Siena from the south.
Strozzi's next plan was for an attack on Florence herself. He aimed to move north-east from Siena towards Arezzo and then advance north down the Arno towards Florence.
This time he was outfought by Merignano. Strozzi was able to get into the Chiani Valley, but he was unable to make any further progress.
A standoff developed near Marciano in the Chiani valley in which Strozzi's men came off worse. Merignano's men had the advantage of height and were able to keep up a damaging artillery bombardment of Strozzi's camp, and were also able to hit their one source of water. Strozzi decided to retreat, and sent some of his cannon on ahead of him.
Strozzi attempted to retreat during daylight on 2 August. Merignano took advantage of the retreat to attack, and inflicted a heavy defeat on the Sienese. Strozzi himself managed to escape from the battle, but the elimination of the Sienese field army meant that Merignano was able to concentrate all of his efforts on the siege of Siena. At first he expected an easy victory, as Strozzi wasn't expected to recover from his wounds and Monluc was ill, but both men recovered and the siege continued.
Monluc conducted a skilful defence of the city, but eventually starvation forced the citizens to surrender. The siege ended in April 1555, although Monluc himself refused to accept defeat and left the city before the final surrender. Siena was given to Florence, although the Emperor kept a number of the Republic's sea ports.