Faced with imminent war with Sweden, the Danish government wanted to recruit the experienced and reputed Schack for a senior position in the Danish army. In order to satisfy Schack's demand for a permanent position, he was in 1658 appointed lieutenant-general, colonel of horse and foot, and made a member of the War Council. He also became naturalized as a Danish noblemen, receiving two manors free of charge for life; Riberhus and Møgeltønder (the later Schackenborg Castle). As military governor, Schack commanded Copenhagen during the Swedish siege of 1659. For his successful defence of the city, he was rewarded with promotion to field-marshal. In the fall of the same year, Schack led the Danish forces to victory over the Swedes at the battle of Nyborg.