 In October 1631 an army, newly recruited by John Frederick, started to reconquer the Prince-Archbishopric — helped by forces from Sweden and the city of Bremen.John Frederick returned to office, only to implement the supremacy of Sweden, insisting that it retain supreme command until the end of the war.With the impending enforcement of the military Major Power of Sweden over the Prince-Archbishopric of Bremen, which was under negotiation at the Treaty of Westphalia, the city of Bremen feared it would fall under Swedish rule too.