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 to relieve
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 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. (1521–1525). The engagements at
Bicocca, the Sesia, and
Pavia are labeled. A Franco-Navarrese force approached the fortress of
Amaiur (
Baztan, Navarre), laying siege to the fortress the Castilians had just reinforced. On 3October 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 after the
Battle of Vaprio d'Adda, retreating to located towns around the
Adda river. 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. == 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. , Constable of France () 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, in which the Republic officially switched sides. 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 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. ==Francis I's campaign in Italy (October 1524 – February 1525)==