After his father's death in 1418, John Jacob received the investiture as Marquis by emperor Sigismund. He distinguished himself for some brilliant military campaign in the Apennines area; he also increased his prestige through the marriage between his sister Sofia and the second last Byzantine Emperor, John VIII Palaiologos.
John Jacob's expansion, however, spurred the reaction of the Dukes of Savoy and of Filippo Maria Visconti of Milan. In 1431 they signed an alliance aiming at the cancellation of the Montferrat state. John Jacob was therefore compelled to ask support to France. In 1432 he was defeated anyway and sued for peace, ceding several territories to Savoy but retaining the seigniory over Montferrat, although with the status of Savoy vassal.
However, when Amadeus VIII asked him the payment of the war expenses, John Jacob rebelled; but, after having besieged in Chivasso, he was forced to cease. His state and his prestige were shattered, and he had again to declare himself vassal of Savoy.
Djurić, Ivan (2009). Il Crepuscolo Di Bisanzio: I tempi di Giovanni VIII Paleologo (in Italian). Donzelli Editore.
de Ganay, Marianne Constrance (1913). Les bienheureuses dominicaines: (1190-1577); d'après des documents inédits (in French). Librairie academique Perrin et Cie.
Soler, Abel (2017). L’Europa cavalleresca i la ficció literària: La cort napolitana d’Alfons el Magnanim: el context de Curial e Guelfa (in Spanish). Vol. III. Publicacions de la Universitat de Valencia.
