[1] Although in previous reigns, younger sons had been granted rich appanages in Switzerland (Genevois, Vaud), Italy (Aosta), or France (Nemours, Bresse), the Savoy dukes found that this inhibited their own aggrandizement while encouraging intra-dynastic strife and regional secession.

[2] Instead of receiving a significant patrimony, Thomas was wed in 1625 to Marie de Bourbon; she was sister to and co-heiress with Louis, Count of Soissons,[3] who would be killed in 1641 while fomenting rebellion against Cardinal Richelieu.

It was arranged that Thomas, as son of a reigning monarch, would hold the rank of first among the princes étrangers at the French court—taking precedence even before the formerly all-powerful House of Guise, whose kinship to the sovereign Duke of Lorraine was more remote.

Marie did eventually inherit her brother's main holding in France, the county of Soissons, but this would be established as a secundogeniture for the French branch of the family.

He managed to rally the remnants at Namur, then retreated before the numerically-superior French and Dutch forces; and he probably served the rest of the campaign with Ferdinand.

In 1638, Thomas served in Spanish Flanders, helping to defend the fortress-city of Saint-Omer against a French siege; in mid-June, he managed to get reinforcements into the place, then with the rest of his small army entrenched about 15 km.

In 1645, now commanding with Du Plessis Praslin, he took Vigevano, and repulsed a Spanish attempt to block his withdrawal at the River Mora, the nearest he ever came to a success in the field.

In 1646, Thomas was put in command of the French expedition sent south to take the Tuscan forts, after which he was to advance further south to Naples, drive out the Spanish and put himself on the throne of the kingdom; but the expedition set off late, and when he besieged Orbetello, the supporting French fleet was defeated by the Spanish and he was forced to raise the siege and conduct a difficult retreat, which he performed so poorly that Cardinal Mazarin subsequently despised his command ability, viewed him as incompetent, and declined to appoint him to the expedition that France sent to support the Naples revolt late in 1647[8] (this did not stop Mazarin from considering him as a potential candidate for a French-backed King of Naples, though Paris was so slow to move on this that Henry II, Duke of Guise was adopted by the Neapolitans instead).

During the Fronde, Thomas linked himself closely with Cardinal Mazarin, who, although effectively prime minister of France, was like him an Italian outsider at the French court.

The Franco-Spanish war had been continuing in north Italy, and late in 1654, increasing Piedmontese hostility to the current French commander Grancey led to a search for a new allied commander-in-chief; the French would have preferred to send the Duke of York (later King James II), but he too was unacceptable to Turin, so Thomas was appointed as joint commander - though his wife was held in France almost as a hostage for his good behaviour.

[10] On 4 April 1655 Thomas Francis commanded the Waldensians to attend Mass or remove to the upper valleys, giving them twenty days in which to sell their lands.

Oliver Cromwell began petitioning on behalf of the Vaudois, and John Milton wrote his famous poem about this, "On the Late Massacre in Piedmont."

In the 1655 campaign, he led an invasion of the Duchy of Milan, though already ill with malaria, and besieged Pavia, where the attack went so badly that he was forced to leave his sick-bed to take direct control of the siege, and even then it had to be raised after nearly two months of fruitless effort.