County of Foix
Archambaud became, co-regent of the County of Foix "suo uxoris", viscount of Béarn, Marsan, Lautrec, and Castelbon, as well as co-prince of Andorra. The King of France, however, objected to this inheritance, as one of the largest territories of the Kingdom would fall into the hands of a subject of England. Thereofore the French Connétable de Sancerre marched into the County of Foix with an army and occupied large parts of it. Isabelle and Archambaud could not counter this threat alone and showed themselves ready to subject themselves to French authority. In the Treaty of Tarbes, 10 May 1399, Isabelle and Archambaud recognized the French King as their feudal lord for the County of Foix, Archambaud cancelled his allegiance to England, and the couple sent their two eldest sons as hostages to the royal court at Paris.