Italian War of 1521–1526 participant Papal States
Initial moves (June 1521 – May 1522)
On 20 August 1521, the Imperial army under Henry of Nassau invaded northeastern France—an attack made in response to de Marck’s attack on Luxembourg. Ardres was overrun, Mouzon was severely damaged after being besieged, and Aubenton was sacked and its inhabitants massacred. The attackers were delayed during the three-week siege of Mézières by the resistance of the French, led by Pierre Terrail, seigneur de Bayard. Francis had time to raise a large army at Reims for relieving Mézières. The town was resupplied a few days before the king's army arrived at the town on 26 September, which was by then largely destroyed. Nassau was forced to withdraw, laying waste to towns along his route of his retreat.Tournai, which had been returned to France by Henry VIII in February 1519 as part of the terms of the Treaty of London, was besieged by Imperial forces. Tournai was left to surrender to the besiegers after Francis's army was ordered to retreat, and later disbanded.
A Franco-Navarrese force approached the fortress of Amaiur (Baztan, Navarre), laying siege to the fortress the Castilians had just reinforced. On 3 October 1521 the Castilians capitulated in exchange for free passage to Castile. The troops of Guillaume Gouffier then headed to Labourd and on to Behobia, capturing the fortress of Urantzu. Fuenterrabia, at the mouth of the river Bidasoa on the Franco-Spanish border, was captured later in the month by French-Navarrese troops under Bonnivet and Claude of Lorraine. The French held this advantageous foothold in northern Spain until March 1524.On 28 November 1521 Charles V and Henry VIII signed in secret the Treaty of Bruges. Odet de Foix, Vicomte de Lautrec, the French governor of Milan, was tasked with resisting the Imperial and Papal forces. Lautrec was outmatched by Prospero Colonna, and by late November 1521 had been forced out of Milan and had retreated to located towns around the river Adda. Lautrec's army was reinforced by Swiss mercenaries. Unable to pay them, he had to give in to their demands to engage the Imperial forces immediately.On 27 April 1522, Lautrec attacked Colonna's combined Imperial and Papal army near Milan at the Battle of Bicocca. Lautrec had planned to use his superiority in artillery to his advantage, but the Swiss, impatient to engage the enemy, masked his guns and charged against the entrenched Spanish arquebusiers. In the resulting melee, the Swiss were badly mauled by the Spanish and by a force of landsknechts (Germanic mercenaries). Their morale broken, the Swiss returned to their cantons; Lombardy was abandoned. Colonna and d'Avalos, left unopposed, proceeded to besiege Genoa, capturing the city on 30 May.Albany's advance towards Naples
After the failure of a peace mission proposed by Pope Clement VII, Francis and the pope negotiated an alliance in secret. Negotiations were concluded on 12 December 1524, and a secret treaty was signed by the pope on 5 January. Clement pledged not to assist Charles in exchange for Francis's assistance with the conquest of Naples by giving Albany free passage through his lands.  In return, Francis promised to cede lands and maintain Medici rule in Florence.The pope tried to stop Francis from acting immediately, because of the dangers inherent in fighting during the winter months. Against the pope's advice, and the counsel of his own senior commanders, Francis almost immediately detached a portion of his forces under the Duke of Albany, sending 5,000 infantry and 500 cavalry south to aid the pope in the invasion of Naples. The size of the army grew when it was joined by Papal States recruits, and French infantry led by the Italian condottiero (Italian mercenary captain), Renzo da Ceri.Francis's ploy failed to achieve his aim of leading the Spaniards to abandon northern Italy, as the Imperial commanders ultimately decided not to attack Albany but to concentrate on relieving Pavia. Lannoy attempted to intercept the expedition near Fiorenzuola, but suffered heavy casualties and was forced to return to Lodi by the intervention of the Black Bands of Giovanni de' Medici, which had just entered French service.