[3] He entered the novitiate of the Dominican convent in his native city at the age of seventeen, was sent to Paris to continue his philosophical and theological studies under Albert the Great and had for a fellow-student there, Thomas Aquinas.

Six years later Sienna was put under an interdict for having espoused the cause of the Emperor Frederick II, then at enmity with the Holy See.

The Siennese petitioned Ambrose to plead their cause before the Sovereign Pontiff, and so successfully did he do this that he obtained for his native city full pardon and a renewal of all her privileges.

For a time, he devoted himself to preaching the Eighth Crusade; and later, at the request of Pope Gregory X, caused the studies which the late wars had practically suspended to be resumed in the Dominican convent at Rome.

He restored peace there between Florence and Pisa[1] and also between the dogal republics of Venice and Genoa, another pair of commercial rivals within Italy.