In 1468, Jacques became a guest at the court of Charles the Bold, Duke of Burgundy, and was appointed to important military commands in the Burgundian army.

Taking advantage of the weakness of the Savoyard ducal power Amadeus IX, in 1471 Jacques elevated his appanage into a quasi-principality, headed by a governor.

[1] Jacques took the opportunity of a conflict with his sister-in-law Yolande to grab several fortified places that Amadeus IX had retained, and an arbitration by Bern and Fribourg in 1471 awarded them to him.

However, these different alliances and clever political manoeuvering by King Louis XI eventually led to the loss of all the territories belonging to the Count of Romont in Vaud.

Winter put an end to the military campaign in the Haute-Alsace, but in April 1475 the Swiss Confederacy, led by Bern, raised militia groups against Vaud.

As Jacques de Savoy could not return to defend his territories, the "Swiss bands" had soon overrun them, taking Grandson, Orbe, Montagny and Echallens.

However, as Charles the Bold became successful on the European battlefields, Jacques of Savoy could at last concentrate his attention on his territories in Vaud.

[2] He participated courageously in the Battle of Guinegate in 1479, where he was wounded, helping to beat the army of King Louis XI of France and keeping the County of Flanders for Mary.

In 1484 Jacques of Savoy married Marie de Luxembourg (1462–1546), his niece and the granddaughter of Louis of Luxembourg-Saint-Pol, Count of Saint-Pol.

Jacques received Saint-Pol from King Charles VIII of France, with other fiefs in Flanders and Brabant out of the inheritance of Louis of Luxembourg.