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,[a] while Ferdinand withdrew his troops in response to Swedish intervention in the Thirty Years' War, and both sides agreed a truce in October 1630.

Control of this region allowed the Habsburg rulers of Spain and Austria to threaten France's restive southern provinces of Languedoc and the Dauphiné, as well as protecting the overland supply route known as the Spanish Road.

The legal position was complicated since Montferrat allowed female inheritance while Mantua did not; in addition, while the duchies had effectively been ruled as independent states, technically their status as Imperial fiefs gave the Holy Roman Emperor a measure of control over the succession.

[7] Historian Peter H. Wilson argues "none of the major powers ... was looking for a fight in Italy" and conflict was caused by a combination of events, including miscommunication between Córdoba and Madrid, as well as Nevers' refusal to compromise.

While Spain was always seeking to strengthen its position in northern Italy, Ferdinand wanted to avoid diverting resources from Germany, as well as to assert Imperial authority by deciding the succession question himself.

[8] Since even large states like France struggled to fight simultaneously in multiple theatres, 17th century diplomacy focused on opening new fronts by building alliances against opponents, or freeing resources by ending an existing conflict.

[14] In addition to support from Empress Eleonora, Venice and the Pope, Nevers was also backed by Ferdinand's military commander Wallenstein, who wanted his army to focus on capturing Stralsund in Northern Germany.

[17] Richelieu positioned 18,000 troops on the Savoyard frontier to deter Imperial intervention, but Ferdinand viewed opposing the French in Italy as a higher priority than supporting Spain against the Dutch.

[26] Charles Emmanuel switched sides again and in December joined Spinola at Casale with 6,500 men, leaving 12,500 to guard Piedmont and another 6,000 in Savoy, while Tuscany, Parma and other Spanish allies provided additional recruits and money.

[32] Despite this, the retention of Casale meant Nevers' cause remained alive; on 26 July, Charles Emmanuel died and was succeeded by his son Victor Amadeus, who was married to Christine of France, Louis XIII's younger sister.

[33] On 6 August, Montmorency defeated the Savoyards at Carignano; losses from disease and desertion left him too weak to relieve Casale, but reinforcements under Charles de Schomberg reached the fortress in early October.

The siege works were flooded by heavy rain while the besiegers had been reduced by plague to under 4,000, their casualties including Spinola; on 29 October, the Spanish finally withdrew and the two sides agreed a truce, negotiated by the papal representative Mazarin.

[34] In June 1630, Richelieu dispatched his close aide Father Joseph and Nicolas Brûlart to open peace talks with Ferdinand at Regensburg, where he was attending a meeting of the Imperial Diet.

[36] Such an agreement compromised the entire basis of French foreign policy, which was to weaken the Habsburgs wherever possible, and threatened their alliance with the Pope, for whom liberating Italy from the "Spanish yoke" was a primary objective.

[38] The need to transfer Imperial troops from Italy to face the Swedish threat forced Ferdinand to sign the Treaty of Cherasco with France on 19 June 1631, which confirmed Nevers as Duke of Mantua and Montferrat, in return for minor concessions to Savoy.

Although both sides agreed to withdraw their armies, Nevers and Victor Amadeus allowed French garrisons to remain in Casale and Pinerolo, which meant despite the expenditure of 10 million escudos and thousands of men, Spain gained nothing from the conflict.

[39] The war proved a major foreign policy success for Richelieu by strengthening the French position in Northern Italy, disrupting the Spanish Road and permanently souring relationships between Spain and Austria.