Arguments as to why St Louis diverted his 1270 crusade to Tunis from Jerusalem have been raging ever since the expedition returned to France. Although historians have recently agreed that the diversion was the decision of Louis himself, this consensus has not led to exploration of his reasons for crusading to a north African port city. This essay argues that the diversion to Tunis is best understood in terms of Louis's ideas about conversion in general and his policy towards the Jews of his land in particular. The close parallels between Louis's Jewish policy and the Tunisian strategy suggest that these conversion policies led Louis to Tunis.