Charles VIII was on good terms with the two powers in northern Italy, Milan and Venice, and both had encouraged his claim over the Kingdom of Naples. Thus he assumed he would have their support when he moved against Alfonso II of Naples, especially as the rival claimant was Ferdinand II of Aragon, King of Spain. At the end of August 1494, in a lightning campaign, he used France's powerful modern army, reinforced by a large contingent of Swiss mercenaries, to sweep through Italy, his mobile field artillery train smashing into dust the tall towers of Italy's medieval castles. He was granted free passage through Milan, but was vigorously opposed by Florence, Pope Alexander VI, and Naples.