The elevation of Charles of Spain to Holy Roman Emperor, a position that Francis had desired, led to a collapse of relations between France and the Habsburgs. In 1519, a Spanish invasion of Navarre, nominally a French fief, provided Francis with a pretext for starting a general war; French forces flooded into Italy and began a campaign to drive Charles from Naples. The French were outmatched, however, by the fully developed Spanish tercio tactics, and suffered a series of crippling defeats at Bicocca and Sesia against Spanish troops under Fernando d'Avalos. With Milan itself threatened, Francis personally led a French army into Lombardy in 1525, only to be defeated and captured at the Battle of Pavia; imprisoned in Madrid, Francis was forced to agree to extensive concessions over his Italian territories in the "Treaty of Madrid" (1526).