The emperor's overwhelming administrative difficulties were complicated at every turn by the intricacies of foreign politics. In the first place, Charles was obliged to wage war with France throughout the greater part of his reign; he had inherited a longstanding quarrel with the French kings, to which the rivalry of Francis I for the empire gave a personal aspect. In the second place, and almost as formidable, was the advance of the Turks up the Danube and the increase of Mohammedan naval power in the Mediterranean. Against Protestant Germany a Catholic monarch might hope to rely on papal assistance, and English support might conceivably be enlisted against France. But the popes, who usually disliked the emperor's Italian policy, were not of great aid to him elsewhere; and the English sovereigns had domestic reasons for developing hostility to Charles. A brief sketch of the foreign affairs of Charles may make the situation clear. [Sidenote: Francis I of France and the Reasons for his Wars with the Emperor Charles V] Six years older than Charles, Francis I had succeeded to the French throne in 1515, irresponsible, frivolous, and vain of military reputation. The general political situation of the time,--the gradual inclosure of the French monarchy by a string of Habsburg territories,-- to say nothing of the remarkable contrast between the character of Francis and that of the persevering Charles, made a great conflict inevitable, and definite pretexts were not lacking for an early outbreak of hostilities. (1) Francis revived the claims of the French crown to Naples, although Louis XII had renounced them in 1504. (2) Francis, bent on regaining Milan, which his predecessor had lost in 1512, invaded the duchy and, after winning the brilliant victory of Marignano in the first year of his reign, occupied the city of Milan. Charles subsequently insisted, however, that the duchy was a fief of the Holy Roman Empire and that he was sworn by oath to recover it. (3) Francis asserted the claims of a kinsman to the little kingdom of Navarre, the greater part of which, it will be remembered, had recently [Footnote: In 1512. See above.] been forcibly annexed to Spain. (4) Francis desired to extend his sway over the rich French-speaking provinces of the Netherlands, while Charles was determined not only to prevent further aggressions but to recover the duchy of Burgundy of which his grandmother had been deprived by Louis XI. (5) The outcome of the contest for the imperial crown in 1519 virtually completed the breach between the two rivals. War broke out in 1521, and with few interruptions it was destined to outlast the lives of both Francis and Charles. [Sidenote: The Italian Wars of Charles V and Francis I]