Bonnivet advanced through the Piedmont. The Imperial commander, Prospero Colonna, had only 9,000 men to oppose the French advance and was forced to retreat to Milan. Bonnivet overestimated the size of the Imperial army and moved into winter quarters rather than attacking the city; the Imperial commanders were able to summon 15,000 landsknechts and a large force under Bourbon's command by 28 December, when Charles de Lannoy replaced the dying Colonna. Many of the Swiss now abandoned the French army, and Bonnivet began his withdrawal. The French defeat at the Battle of the Sesia, where Bayard was killed while commanding the French rearguard, again demonstrated the power of massed arquebusiers against more traditional troops; the French army then retreated over the Alps in disarray. D'Avalos and Bourbon crossed the Alps with nearly 11,000 men and invaded Provence in early July 1524. Sweeping through most of the smaller towns unopposed, Bourbon entered the provincial capital of Aix-en-Provence on 9 August 1524, taking the title of Count of Provence and pledging his allegiance to Henry VIII in return for the latter's support against Francis. By mid-August, Bourbon and d'Avalos had besieged Marseille, the only stronghold in Provence that remained in French hands. Their assaults on the city failed and when the French army commanded by Francis himself arrived at Avignon at the end of September 1524, they were forced to retreat back to Italy. Francis I's campaign in Italy (October 1524 – February 1525) French advance into Lombardy On 17 October 1524, as Bourbon and Pescara were returning to Genoa, Francis confirmed his mother as regent during his absence. Shortly afterwards, he crossed the Alps and advanced on Milan at the head of an army numbering more than 40,000. Imperialist troops, not yet recovered from the campaign in Provence, were in no position to offer serious resistance. As the French advanced, the viceroy of Naples retreated from Asti towards Milan. Lannoy, the city's viceroy, who had concentrated 16,000 men to resist the French advance, decided that Milan could not be defended. The city was ridden with the plague, and so to avoid his troops becoming infected, on 26 October he withdrew to Lodi, leaving Milan through one gate as the French vanguard under Salazzo entered through another. Francis installed Louis II de la Trémoille as the city's governor. At the urging of Bonnivet and against the advice of his other senior commanders,