An imperial generalissimo by land, and Admiral of the Baltic Sea from 21 April 1628, who had made himself ruler of the lands of the Duchy of Friedland in northern Bohemia, Wallenstein found himself released from service on 13 August 1630 after Ferdinand grew wary of his ambition. Several Protestant victories over Catholic armies induced Ferdinand to recall Wallenstein, who again turned the war in favor of the Imperial cause. Dissatisfied with the Emperor's treatment of him, Wallenstein considered allying with the Protestants. However, he was assassinated at Cheb in Bohemia by one of the army's officials, Walter Devereux, with the emperor's approval.