On 10 May 1543, the skilled and ambitious man backed by Emperor Charles V was elected Prince-Bishop of Augsburg; in 1544 he was appointed Cardinal of the "titulus" of Santa Balbina by Pope Paul III for settling the dispute with the Emperor upon the concessions he had made to several Lutheran princes of the Schmalkaldic League at the Fourth Diet of Speyer. Otto however urged on a military expedition against the revolting nobles, which the Emperor prepared by forging an alliance with the (Lutheran) Duke Maurice of Saxony, when the Protestants in 1546 sparked the Schmalkaldic War by a preventive strike against the town of Füssen, a possession of the Augsburg bishop.