Italian War of 1521–1526 participant Holy Roman Empire
France at bay
The loss of Lombardy was followed by England entering openly into the conflict, when on 29 May 1522, the English formally declared war on France. Henry VIII and Charles signed the Treaty of Windsor on 16 June 1522. The treaty outlined a joint English-Imperial attack against France. Charles agreed to compensate England for the pensions that would be lost because of conflict with France and to pay the past debts that would be forfeit; to seal the alliance, he also agreed to marry Henry's only daughter, Mary. In July, the English raided Morlaix and in September an English army marched from Calais, burning and looting the countryside in an unsuccessful attempt to engage the French in battle.
To raise money, Francis pursued a lawsuit against Charles III, Duke of Bourbon, who had received the majority of his holdings through his marriage to Suzanne, Duchess of Bourbon. After Suzanne's death, Louise of Savoy, her sister and the king's mother, insisted that the territories in question should pass to her because of her closer kinship to the deceased. Francis was confident that seizing the disputed lands would improve his own financial position sufficiently to continue the war and began to confiscate portions of them in Louise's name. Bourbon, angered by this treatment and increasingly isolated at court, sought redress by making overtures to Charles V.The death of Doge Antonio Grimani brought Andrea Gritti, a veteran of the War of the League of Cambrai, to power in Venice. He quickly began negotiations with the Emperor and on 29 July 1523 concluded the Treaty of Worms, which removed the Republic from the war. Bourbon continued his scheming with Charles, offering to begin a rebellion against Francis in exchange for money and German troops. When Francis, who was aware of the plot, summoned him to Lyon in October, he feigned illness and failed to appear. Francis ordered as many of Bourbon's associates as could be captured to be brought to justice after the Duke reached Imperial territory and openly entered the Emperor's service.Charles then invaded southern France over the Pyrenees. Lautrec successfully defended Bayonne against the Spanish, but Charles was able to recapture Fuenterrabia in February 1524. On 19 September 1523, an English army under the Duke of Suffolk advanced into Picardy from Calais. The French, stretched thin by the Imperial attack, were unable to resist, and Suffolk soon advanced past the Somme, devastating the countryside in his wake and stopping only 80 kilometres (50 mi) from Paris. When Charles failed to support the English offensive, Suffolk—unwilling to risk an attack on the French capital—turned away from Paris on 30 October, returning to Calais by mid-December.Francis now turned his attention to Lombardy.  In September 1523, a French army under 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.