Thomas' first recorded service is as a commander in the Piedmontese army under his father in the war against France in 1630 (see War of the Mantuan Succession). It was probably around this time that he first encountered Mazarin, who (though his public position was quite complex) was during 1630-32 in effect a French agent at the Piedmontese court. When the new Duke Victor Amadeus I was forced to accept a French occupation of Pinerolo (Peace of Cherasco, 26 April 1631, and associated secret agreements, implemented 1632), there was widespread dissatisfaction in Piedmont, and Thomas, with his brother Maurice, went to join the Spanish, at which Victor Amadeus confiscated their revenues. (The exact date of the move is unstated, but was probably 1632, certainly no later than 1634.) Though welcomed by the Spanish given that he was related to both the French and Spanish royal families, Thomas was not entirely trusted by them, and had to send his wife and children to Madrid as hostages.