War of the Mantuan Succession The War of the Mantuan Succession ( 1628 – 31 ) was a peripheral part of the Thirty Years ' War . Its " casus belli " was the extinction of the direct male line of the House of Gonzaga in December 1627 . Brothers Francesco IV ( 1612 ) , Ferdinando ( 1612 – 26 ) and Vincenzo II ( 1626 – 27 ) , the last three dukes of Gonzaga , had all died leaving no legitimate heirs . The war , fought among the backers of rival claimants , pitted France against the Habsburgs in a contest for control of northern Italy . Background . Mantua is the ancestral city where the male line of the Gonzaga dynasty ruled , first as marquess es , then after 1540 as dukes , in vassalage to the Holy Roman Empire . Monferrato was a duchy since 1574 on the eastern side of Piedmont , and an imperial fief since the eleventh or twelfth century . The Gonzagas had enlarged their realm with Monferrato after receiving it in dowry from the wife of duke Frederick II Gonzaga . On 22 September 1612 , Francis IV , Duke of Mantua and Monferrato died at the age of 26 . His death occurred only a couple of months after the death of his father Vincent I , Duke of Mantua . He left only a three - year old daughter , Maria of Mantua . Francis IV also had two younger brothers who , both being cardinals of the Church and consecrated priests , could not marry and were thus ineligible to succeed to Mantua ' s throne . Nonetheless , Francis ' two brothers , Ferdinando I ( 1587 – 1626 ) and Vincenzo II ( 1594 – 1627 ) , eventually did become Dukes of Mantua . Despite hastily marrying via " ecclesiastical revisions , " neither produced any legitimate children . A crisis erupted when Vincenzo II died on 26 December 1627 at the age of 33 , the same day that his niece Maria Gonzaga ' s marriage with Charles de Nevers was celebrated . Nevers was the eldest son and heir of Charles , Duke of Nevers , Rethel and Mayenne . Moreover , he was the head of the cadet branch of the House of Gonzaga , and after Vincenzo II , heir male of the Duchy of Mantua . The claimants , and their supporters . The Duke of Nevers was a son of Luigi , younger brother of Vincenzo II ' s grandfather ( see family tree ) . 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 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 ( 1612 – 1660 ) 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 ( 1598 – 1655 ) , 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 Spanish - Imperial 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 . Conflict . The initial attempt of Don Gonzalo Fernandez de Córdoba , Spanish governor of Milan , and Charles - Emmanuel was to partition the Mantuan - Montferrat patrimony , which lay to east and to west of Milan . The Spanish minister supported the Guastalla claimant in Mantua , as the weaker of two neighbors , and the Savoy claimant in Montferrat , the lesser of the territories . Friction between the confederates ensued , when Charles - Emmanuel moved his troops into more territory than had been agreed upon , laying siege to the town of Casale , capital of Montferrat . The French , although Louis XIII of France and Cardinal Richelieu were concerned at home with Huguenot uprisings in Languedoc after the fall of La Rochelle in 1628 , sent forces to relieve Casale near the border with Milanese territory , besieged by a Habsburg army from Milan . The French forces crossed the Alps in March 1629 , forced Susa in Piedmont , on 6 March , delivered the siege of Casale on 18 March 1629 and took the fortress of Pinerolo on 30 March 1630 . In the meantime the short - lived Treaty of Susa was signed with the Duke of Savoy in April 1629 , whereupon they returned to France , leaving behind a small garrison . The papal envoy in negotiations at Casale was Jules Mazarin . Emperor Ferdinand II ' s forces under Ramboldo , Count of Collalto invaded the Grisons and Valtelline . The governor was recalled from Milan , followed by the insults of the citizens , for bread had been scarce for months . The following winter , Milan was devastated by the bubonic plague introduced by the armies , which has been vividly described by Manzoni . Later in 1629 , Emperor Ferdinand II sent a Landsknecht army to besiege Mantua . Charles left without the promised support from Louis XIII of France . The siege lasted until July 1630 , when the city , already struck by a plague , was brutally put to the sack for three days and three nights by troops led by Count Aldringen and Gallas . But the Emperor did not succeed in Mantua . Due to developments in Germany , where the Swedes were warring , he was forced to return his attention to the principal theatre of the big war . The Peace of Regensburg ( 1630 ) . The French first agreed to the Peace of Regensburg ( or the Treaty of Ratisbonne ) , which was negotiated by French representatives Father Joseph and Nicolas Brûlart de Sillery . The accord was signed on 13 October 1630 , which provided favorable terms to French interests in Italy despite their military setbacks . Specifically , the French were allowed to maintain their garrison in Grisons . The accord also confirmed Charles Gonzaga - Nevers as Duke of Mantua and Marquess of Monterrat in exchange for minor concessions to Charles Emmanuel of Savoy and Ferrante of Guastalla . The Habsburgs would on their side reduce their number of troops in the region . The treaty was seen as so unfavorable to the Spanish that the Spanish prime minister , Olivares , considered it no different than a surrender . The treaty , moreover , contained a troublesome clause . It included an agreement whereby the French were not permitted to establish alliances in Germany against a reigning Holy Roman Emperor . This should have sidelined France in the ongoing conflict . Louis XIII of France refused to accept this , and the Austrians found themselves still at war , yet with diminished forces in the area . The new forces sent south of the Alps were to be sorely missed when Swedish forces under Gustavus II Adolphus invaded from the north . The Treaty of Cherasco ( 1631 ) . The Italian peace was eventually made with the Treaty of Cherasco , signed in a city in Piedmont on 19 June 1631 . France , which in 1629 had taken Savoy , then captured Pinerolo in Piedmont the following year , renounced its conquests in Italy . Charles Gonzaga - Nevers was confirmed as ruler in Mantua and Montferrat , with concessions to the other claimants : Vittorio Amedeo I , who succeeded in Savoy after the sudden death of his father , Duke Charles Emmanuel , gained Trino and Alba in Montferrat ; while Cesare II of Guastalla , Ferrante ' s son , was given Luzzara and Reggiolo . Later it was discovered that by a secret treaty with Vittorio Amedeo , Pinerolo was surrendered to France .