of opposition to the central authorities. While Spain remained the dominant power in Lombardy and in Southern Italy, its reliance on long exterior lines of communication was a potential weakness. This applied particularly to the Spanish Road, which allowed them to safely move recruits and supplies from the Kingdom of Naples through Lombardy to their army in Flanders. The French sought to disrupt the Road by attacking the Spanish-held Duchy of Milan or blocking the Alpine passes through alliances with the Grisons. Montferrat and its fortress of Casale Monferrato were subsidiary territories of the Duchy of Mantua and their possession allowed the holder to threaten Milan. This 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. While a relatively minor conflict, the struggle 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. In March 1629, the French stormed Savoyard positions in the Pas de Suse, lifted the Spanish siege of Casale and captured Pinerolo. The Treaty of Suza then ceded the two fortresses to France and allowed their troops unrestricted passage through Savoyard territory, giving them control over Piedmont and the Alpine passes into Southern France. However, as soon as the main French army withdrew in late 1629, the Spanish and Savoyards besieged Casale once again, while Ferdinand II provided German mercenaries to support a Spanish offensive which routed the main Venetian field army and forced Nevers to abandon Mantua. By October 1630, the French position seemed so precarious their representatives agreed the Treaty of Ratisbon but since the terms effectively destroyed Richelieu's policy of opposing Habsburg expansion, it was never ratified. Several factors restored the French position in Northern Italy, notably a devastating outbreak of plague; between 1629 and 1631, over 60,000 died in Milan and 46,000 in Venice, with proportionate losses elsewhere. Richelieu took advantage of the diversion of Imperial resources from Germany to fund a Swedish invasion,