Dumas failed an attempt to conquer Egypt and the Levant on the "Expédition d’Égypte" when he was a commander of the French cavalry forces. On the march from Alexandria to Cairo, he clashed verbally with the Expedition's supreme commander Napoleon Bonaparte, under whom he had served in the Italian campaigns. In March 1799, Dumas left Egypt on an unsound vessel, which was forced to put aground in the southern Italian Kingdom of Naples, where he was taken prisoner and thrown into a dungeon. He languished there until spring of 1801.