François I had a dream, a single goal that motivated the foreign policy of his entire reign: the recovery of the duchy of Milan, a region of Italy he had inherited from his great-grandmother, Valentina Visconti, and which had been lost by his predecessor, Louis XII. In 1515, very first year of his reign, François launched an expedition into Italy and succeeded in securing Milan after a stunning victory at Marignano. It was an auspicious beginning which brought him much glory and established his reputation as roi-chevalier, or knightly king.
However, to retain control in Milan, the French needed the support and cooperation of the Pope, Leo X. François initially received Leo's support, but in 1520, the Hapsburg prince Charles V was elected Holy Roman Emperor and Leo signed a treaty of alliance with him instead. In August 1521 the Emperor (who controlled Spain and Flanders and was promised Naples by the Pope) attacked the north-east border of France. The first of an ongoing series of wars between François and Charles began.
After a significant loss to imperial forces at La Bicocca in April 1522, the French lost much of the area they had seized; only the castle of Milan and Cremona remained in French hands. Henry VIII soon entered the war on the imperial side. The rebellion of the duke Charles de Bourbon, Constable of France (a story worthy of its own post), complicated things immensely for François, for Bourbon entered into league with the Emperor and the English king. François's enemies plotted to attack on three fronts. When Bourbon was routed from Marseilles in 1524, François saw the opportunity to invade Italy, a plan he had deferred for years. He crossed the Alps at the head of his troops, determined to recreate his earlier success at Marignano.
Milan had been abandoned by the imperialist troops, so the French army followed them to the city of Pavia, a heavily fortified town whose garrison was commanded by one of the best generals of the day. For four months the French lay siege to the town but refused to be drawn into the open. Finally, during the night of 23 February 1525, the imperial forces staged a surprise attack on the French, who were camped in the walled park of Mirabello. François, forewarned, charged at the head of his cavalry but got in the way of his own guns. This blunder destroyed any chance of victory and the French troops, exposed to imperial arquebusiers and left shorthanded by the failure of their Swiss mercenaries to arrive, were quickly decimated. The French noblemen fought bravely, many in hand-to-hand combat on horseback, but were unable to hold back the imperial tide.
François himself was surrounded by imperial soldiers, each of whom wanted to claim the honor of capturing him. A steward of Bourbon, La Mothe, finally took him prisoner, but François refused to surrender until Charles de Lannoy, the Viceroy of Naples (who shared command of the imperial army with the duc de Bourbon), arrived. To him François gave up his sword, along with his dream of another glorious Italian victory. Adding to the blow was the loss of many of the king's closest childhood friends and comrades in the battle. Topping everything, François, along with Anne de Montmorency and a dozen or so other high-ranking noblemen, became prisoners of his arch-enemy, the Holy Roman Emperor Charles V. François's captivity at Charles's hands would last more than a year and permanently color relations between the two rulers ever after.
[Source: R. J. Knecht, Renaissance Warrior and Patron, chapters 8-11]
Tapestry of the Battle of Pavia