County of Foix
As a result of the Treaty of Tarbes signed on 10 May 1399, John's parents were given possession of the County of Foix, but it required that his father renounce his English alliance, and it stipulated that John and one of his brothers were to be sent as hostages to the Royal Court of France to ensure observance of the terms of the treaty. On 24 April 1406 Charles VI of France recognised John as heir to the County of Foix and he was also involved in several operations against the English: the Siege of Bordeaux in 1404-1405 and the Seat of Blaye in 1406. In 1409, John accompanied King Martin of Aragon on an expedition to Sardinia against the Genoese at the Battle of Sanluri on 30 May 1409 and returned to Foix in September.