In 1189, Boniface joined the council of regency for Thomas I of Savoy, son of his cousin Humbert III, until the boy came of age about two years later. In 1191, after the new Emperor Henry VI granted him the county of Incisa, a fifteen-year war broke out against the neighbouring communes of Asti and Alessandria. Boniface joined the Cremona League, while the two cities joined the League of Milan. Boniface defeated the cities at Montiglio in June that year, but the war as a whole went badly for the dynasty's interests. At Quarto, he and Vaqueiras saved his brother-in-law Alberto of Malaspina when he was unhorsed. The first phase of the war ended with a truce in April 1193. By now, Boniface was Marquis of Montferrat, following the deaths of his father in 1191 and of Conrad, the newly elected king of Jerusalem, in 1192. No claim to Montferrat ever seems to have been made on behalf of Conrad's posthumous daughter, Maria.