5. Charles V.--The conquest of Lombardy made France the greatest power in Christendom; but its king was soon to find a mighty and active rival. The old hatred between France and Burgundy again awoke. Mary of Burgundy, the daughter of Charles the Bold, had married Maximilian, Archduke of Austria and King of the Romans, though never actually crowned Emperor. Their son, Philip, married Juana, the daughter of Ferdinand, and heiress of Spain, who lost her senses from grief on Philip's untimely death; and thus the direct heir to Spain, Austria, and the Netherlands, was Charles, her eldest son. On the death of Maximilian in 1518, Francis proposed himself to the electors as Emperor, but failed, in spite of bribery. Charles was chosen, and from that time Francis pursued him with unceasing hatred. The claims to Milan and Naples were renewed. Francis sent troops to occupy Milan, and was following them himself; but the most powerful of all his nobles, the Duke of Bourbon, Constable of France, had been alienated by an injustice perpetrated on him in favour of the king's mother, and deserted to the Spaniards, offering to assist them and the English in dividing France, while he reserved for himself Provence. His desertion hindered Francis from sending support to the troops in Milan, who were forced to retreat. Bayard was shot in the spine while defending the rear-guard, and was left to die under a tree. The utmost honour was shown him by the Spaniards; but when Bourbon came near him, he bade him take pity, not on one who was dying as a true soldier, but on himself as a traitor to king and country. When the French, in 1525, invaded Lombardy, Francis suffered a terrible defeat at Pavia, and was carried a prisoner to Madrid, where he remained for a year, and was only set free on making a treaty by which he was to give up all claims in Italy both to Naples and Milan, also the county of Burgundy and the suzerainty of those Flemish counties which had been fiefs of the French crown, as well as to surrender his two sons as hostages for the performance of the conditions.