A subsidiary territory of the Duchy of Mantua was Montferrat and its fortress of Casale Monferrato, whose possession allowed the holder to threaten Milan. Its importance meant when the last duke in the direct line died in December 1627, France and Spain backed rival claimants, resulting in the 1628 to 1631 War of the Mantuan Succession. The French-born Duke of Nevers was backed by France and the Republic of Venice, his rival the Duke of Guastalla by Spain, Ferdinand II, Savoy and Tuscany. This minor conflict had a disproportionate impact on the Thirty Years War, since Pope Urban VIII viewed Habsburg expansion in Italy as a threat to the Papal States. The result was to divide the Catholic church, alienate the Pope from Ferdinand II and make it acceptable for France to employ Protestant allies against him.