context: The Duke of Nevers was a son of Luigi, younger brother of Vincenzo II's grandfather . Luigi had been naturalized French as Louis about 1550, and married the heiress of the duchies of Nevers and Rethel in 1566. For the French Crown Nevers, a French peer, would naturally be preferable as ruler in Mantua. Nevers arrived there in January 1628 and was proclaimed its sovereign. There were two rival claimants. One was Charles Emmanuel I, Duke of Savoy, whose daughter Margaret was the widow of Francis IV. Although their son had died an infant in 1612, it was their elder daughter Maria  who had married Charles de Nevers in 1627. Charles Emmanuel based his right to Mantua on his daughter's claim to a substantial portion of the Gonzaga realm, the duchy of Montferrat, which was demonstrably heritable by females since the Gonzagas had acquired it through marriage to Margherita Paleologa in 1540. The other claimant was Ferrante II, Duke of Guastalla, a distant Gonzaga cousin who voiced his claim but did not immediately place troops in the field. He was, however, supported by Emperor Ferdinand II, whose wife at the time, Eleanor of Mantua the elder , had been the sister of the last three Dukes of Mantua. He sought to re-attach the Duchy of Mantua to the Holy Roman Empire; Ferrante being in the Imperial-Spanish camp, was a useful tool to that purpose. But as the Thirty Years' War wore on, it affected dynastic alliances. Charles Emmanuel obtained support from the Habsburgs, who controlled Milan. The resulting French-Habsburg war over the succession was just one of the many theatres of the Thirty Years' War, fought all over Europe.
question: Who controlled Milan
answer: Habsburgs