When Thomas Francis, Prince of Carignano and his Piedmontese faction captured Turin, the French garrison supporting the Regent Christine Marie of France retired within the citadel and continued to resist.

Finally, a Spanish army under Diego Felipez de Guzmán, Marquis of Leganés appeared and encircled the French besiegers.

On 31 May a Spanish army under the governor of Milan, Diego Felipez de Guzmán, Marquis of Leganés, approached, but despite his numerical superiority dared not make a full-scale assault on Harcourt's lines, so he himself began to entrench around the French, starting a weeks-long conflict over individual posts but, despite constant pressure from Prince Thomas, only once risking an outright battle, in mid-July (14 July according to Saluzzo, 11 July according to Hanotaux and Bérenger), when he was thoroughly repulsed.

The situation was particularly complicated in that Prince Thomas in Turin continued for weeks to hold some posts outside the city, ensuring some contact with the Spanish and a line of communication for supplies; the last such post fell to Harcourt late in July, and after this, food shortages began to hit the population and garrison - forage for horses ran out first, and Thomas twice attempted sorties, on 23 and 31 July, intended to enable his cavalry to escape, but failing both times.

Turin eventually had to consider surrender, and Prince Thomas, who had maintained on-off talks with the French and the Regent throughout the siege, opened negotiations with Harcourt, who made an agreement contrary to orders - if the French historian Hanotaux is to be believed, he was unaware of orders en route from Paris that he was to accept nothing from Thomas except pure surrender as a prisoner or agreement to enter French service; according to Saluzzo, Harcourt knew that Cardinal Richelieu's agent Mazarin was on his way with orders but wanted to avoid sharing the glory of a victory with anyone else so hastened to reach a settlement before Mazarin arrived.