Social integration allowed Jews to make great advances in new fields, including mathematics, astronomy, philosophy, chemistry and philology, with some even gaining political power under Islamic rule. For example, the vizier of Baghdad entrusted his capital to Jewish bankers, Jews were put in charge of certain parts of maritime and slave trade, and Siraf, the principal port of the caliphate in the 10th century, had a Jewish governor. Increased commercial freedom increased their integration into the Arab marketplace. Leon Poliakov writes that in the early ages of Islam, Jews enjoyed great privileges, and their communities prospered. No laws or social barriers restricted their commercial activities, and exclusive trade and craft guilds like those in Europe did not exist. Jews who moved to Muslim lands found themselves free to engage in any profession, resulting in less stigma than in Europe where such restrictions were still in force. This, coupled with more intense Christian persecution, encouraged many Jews to migrate to areas newly conquered by Muslims and establish communities there.