The War of the Mantuan Succession (1628–1631) was a related conflict of the Thirty Years' War, caused by the death in December 1627 of Vincenzo II, last male heir in the direct line of the House of Gonzaga and ruler of the duchies of Mantua and Montferrat. These territories were key to control of the Spanish Road, an overland route that allowed Habsburg Spain to move recruits and supplies from Italy to their army in Flanders. The result was a proxy war between France, who supported the French-born Duke of Nevers, and Spain, who backed his distant cousin the Duke of Guastalla.
Fighting centred on the fortress of Casale Monferrato, which the Spanish besieged twice, from March 1628 to April 1629 and from September 1629 to October 1630. French intervention on behalf of Nevers in April 1629 led Emperor Ferdinand II to support Spain by transferring Imperial troops from Northern Germany, who captured Mantua in July 1630. However, French reinforcements enabled Nevers to retain Casale, while Ferdinand withdrew his troops in response to Swedish intervention in the Thirty Years' War, and the two sides agreed a truce in October 1630.
The June 1631 Treaty of Cherasco confirmed Nevers as Duke of Mantua and Montferrat, in return for minor territorial losses. More importantly, it left France in possession of Pinerolo and Casale, key fortresses which controlled access to passes through the Alps and protected their southern borders. The diversion of Imperial and Spanish resources from Germany allowed the Swedes to establish themselves within the Holy Roman Empire and was one reason for the Thirty Years' War continuing until 1648.