Italy was the main theater of the combat. In the first stage, the imperial forces, with the aid of a papal army, speedily drove the French garrison out of Milan. The Sforza family was duly invested with the duchy as a fief of the Empire, and the pope was compensated by the addition of Parma and Piacenza to the Patrimony of Saint Peter. The victorious Imperialists then pressed across the Alps and besieged Marseilles. Francis, who had been detained by domestic troubles in France, [Footnote: These troubles related to the disposition of the important landed estates of the Bourbon family. The duke of Bourbon, who was constable of France, felt himself injured by the king and accordingly deserted to the emperor.] now succeeded in raising the siege and pursued the retreating enemy to Milan. Instead of following up his advantage by promptly attacking the main army of the Imperialists, the French king dispatched a part of his force to Naples, and with the other turned aside to blockade the city of Pavia. This blunder enabled the Imperialists to reform their ranks and to march towards Pavia in order to join the besieged. Here on 24 February, 1525,--the emperor's twenty-fifth birthday,--the army of Charles won an overwhelming victory. Eight thousand French soldiers fell on the field that day, and Francis, who had been in the thick of the fight, was compelled to surrender. "No thing in the world is left me save my honor and my life," wrote the king to his mother. Everything seemed auspicious for the cause of Charles. Francis, after a brief captivity in Spain, was released on condition that he would surrender all claims to Burgundy, the Netherlands, and Italy, and would marry the emperor's sister. [Sidenote: The Sack of Rome, 1527]