As a young man, Schack began his military career by serving in the Danish army during the early years of the Thirty Years' War. He entered Swedish service in 1630, and thence in French in 1635, becoming colonel of cavalry 1642, and later maréchal de camp. In 1651 Schack retired from the French service to his estates in Saxe-Lauenburg, where he remained until 1656, when he became military commandant of Hamburg.
Danish service
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 chargefor life; Riberhus and Møgeltønder. 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.
Political role
Schack was a popular figure in Copenhagen, not only as a defender of the city against the Swedes, but as a defender of its privileges. In 1660 he was co-opted to the Danish Rigsråd, in spite of being a recently naturalized foreigner. When Frederick III in the same year overthrow the Danish constitution, and created an absolute monarchy, Schack was, together with Hannibal Sehested, the King's main supporter in the Rigsråd, persuading the clergy and bourgeoisie to support the regime change. The King needed loyal supporters in the government, and Schack was in 1660 appointed to Rigsfeltherre, and made president of the Board of War, a member of the Board of State, and Stiftsbefalingsmand of Ribe stift. In 1670 he was also appointed a member of the Gehejmekonseil, which that year was created as a successor to the Rigsråd. Within the new government, he was the leading proponent of the military party, in opposition to Sehested and the Treasury, and an efficient administrator of military affairs. As a reward, he was in 1661 given Møgeltønder in fee simple, and became one of the first enfeoffed counts, Lensgreve, when the King in 1671 created that dignity.
