Giovanni became a "condottiero", or mercenary military captain, in the employ of Pope Leo X (Giovanni di Lorenzo de' Medici) and on March 5, 1516 led the war against Francesco Maria I della Rovere, Duke of Urbino. He thenceforth formed a company of his own, mounted on light horses and specializing in fast but devastating skirmishing tactics and ambushes. In 1520 he defeated several rebel barons in the Marche. The following year Leo X allied with Emperor Charles V against King Francis I of France to regain Milan, Parma and Piacenza; Giovanni was called in under the command of Prospero Colonna, defeating the French at Vaprio d'Adda in November.