The Third Hapsburg-Valois War (1536-38) was an inconclusive clash triggered by the death of the last Sforza duke of Milan, and that ended as a minor French victory (Italian Wars, 1494-1559).
At the end of the Second Hapsburg-Valois War (1526-30) Charles V had become the dominant figure in Italy. He had come to peace with Pope Clement VII (at least temporarily) and had been crowned as Emperor at Bologna. He remained in secure possession of Naples and Sicily and Milan was to come to him after the death of Duke Francesco Sforza. Florence was ruled by the Pope's Medici family and Genoa by Charles's ally Andrea Doria.
As always the main threat to Charles's position came from Francis I of France, who was unwilling to accept his defeat in the First or Second Hapsburg-Valois Wars or the terms of the Treaty of Cambrai of 1529. The peace with Clement VII was fragile at best, and the divorce from Catherine of Aragon had damaged his relationship with Henry VIII of England.
Clement and Francis began to plot against the peace settlement as early as 1531. On 9 June 1531 Clement arranged the marriage of his relative Catherine de Medici to Henry, Duke of Orleans, the second son of Francis I and future Henry II of France. In return the Pope promised to support Francis's claim to Milan and Genoa. Catherine was delivered to her new husband at Marseilles in October 1533.
In the following year Francis made his intentions clear. In September he suggested a double marriage alliance between the houses of Hapsburg and Valois, but the terms he wished to impose showed that the suggestion was never serious. Milan, Asti and Genoa were to come to France and Charles was to make peace with Francis's Protestant allies in Germany. The same month saw the death of Pope Clement, and his replacement by Alessandro Farnese, Pope Paul III.
Charles may now have expected another war with France, but first he wanted to deal with the Barbary Pirates of Tunis. Originally based at Algiers, they had expanding into Tunisia under Hayreddin Barbarossa II, and in July 1534 carried out a massive raid on the Italian coast. Barbarossa was also officially an Admiral of the Ottoman Fleet, and as such posed a real threat to the Christian position in the Mediterranean.
In May 1535 Charles set sail, supported by Andrea Doria of Genoa, Portugal, the Knights of Malta, Venice and Pope Paul III. His fleet landed at Carthage and after a short campaign captured Tunis. The previous Muslim ruler of the city was restored, although Spain kept a few footholds in the area, and Charles then returned to Sicily, where he landed on 17 August 1535.
On 1 November 1535 Francesco Sforza died. Under the terms of the Treaty of Cambrai Milan now passed to Charles, who offered it to Charles of Angouleme (then third in line to the French throne), as long as he married Sforza's widow and Charles's niece Christina of Denmark. In return Francis would have to support Charles on a wide range of issues, including the election of his brother Ferdinand as King of the Romans, or heir to the Holy Roman Empire. The young prince, who was then 13, would have been placed under Charles's control, and Imperial garrisons would have remained in parts of Milan. Unsurprisingly Francis didn't accept this offer and instead prepared for war. Charles was unwilling to offer Milan either of Charles's older brothers. The Dauphin was excluded because that would mean the duchy would be united with France. The Duke of Orleans was excluded by his marriage into the Medici family, and by the poor health of the Dauphin, which meant that he was likely to inherit the throne, once again uniting Milan with France.
The first target for the French was Savoy. In earlier wars Savoy had been a French ally, but the current duke, Charles III, was closely tied to Charles by marriage and as part of the imperial defensive league. Since 1530 Charles had been at war with Geneva, which had rebelled against his rule, and early in 1536 that war turned against him.
Francis now claimed part of the Duchy, and in March the French invaded. They captured Turin and all of Savoy. This marked the start of open warfare between Francis and Charles V. Imperial troops occupied as many strong points in Piedmont as they could reach before the French. The news that fighting had broken out reached Charles as he was moving north from Naples to Rome.
Charles reached Rome on 5 April 1536. While he was there he asked the Pope to decide the case between himself and Francis, and also issued a second challenge to a duel between the two men. Neither idea was practical, and so Charles prepared for a two-pronged invasion of France. He led one army into Provence, while a second army, under the Count of Nassau, was to invade Picardy.
Neither invasion was a success. Charles crossed into France on 25 July, but found that Anne of Montmorency was determined to fight a defensive war. The French were camped at Avignon and Valence on the Rhone. Charles advanced to Aix-le-Provence, and threatened Marseille but was unable to take the place, Finally, on 13 September, he was forced to order a retreat back into Italy.
The invasion of Picardy made a little more progress. Guise was captured, but the army then got stuck outside Peronne, and in September Nassau also had to retreat.
Early in 1537 Francis announced that he was resuming control of Flanders and Artois, two areas that he had surrendered in the Treaty of Madrid of 1526. In March he invaded Artois and captured a number of places. He then began to build a fortification at Saint-Pol, but withdrew before it was completed. The Estates of the Netherlands were roused by this attack and provided funding for an army that attacked and destroyed the incomplete fortress at Saint-Pol (June 1537). The Imperial army then moved north to besiege Therouanne, but this fortress held out until 30 July, when a ten month armistice was agreed in the Netherlands and north-eastern France.
The final campaign of the war came in October 1537 when Montmorency invaded Savoy, forcing Charles's men to evacuate Turin and Pinerolo, but in the following month an armistice was agreed. The actual peace negotiations took some time, but the Truce of Nice was finally agreed on 17 June 1538. This was a ten-year long truce that effectively recognised the situation at the end of the fighting. Francis was thus left in possession of most of Artois, Savoy and Piedmont.
This peace lasted for four years, before the outbreak of the Fourth Hapsburg-Valois War of 1542-44. This Fourth War would be the last war fought between Charles and Francis, but only because Francis died in 1547, before the outbreak of the Fifth and Final Hapsburg-Valois War.