Verden even conflicted on the border between each other — i.e. on who may levy taxes where — which were not solved, even though the two fiefs were ruled in personal union by Sweden. Emperor Ferdinand III at first enfeoffed the Queen regnant Christina of Sweden and her legal heirs with the duchies, as Sweden's reward from its participation in the Thirty Years' War. Bremen-Verden provided Sweden a strategic advantage, because it would participate with them in recruiting and financing armies in two imperial circles already covering all of the northern and north-western parts of the Holy Roman Empire, with Swedish Pomerania, a member in the Upper Saxon Circle, covering the Empire's North East. Personal union with Sweden (1648–1712) and under Danish occupation (1712–1715) The Swedes installed a new authority, Bremen-Verden's General Government (), and chose Stade to be the new seat of government, with Bremervörde being the former Bremian capital, and Rotenburg upon Wümme being the former capital of the Prince-Bishopric of Verden. The Swedish takeover in 1648 became a milestone for Bremen-Verden's interior constitution. Bremen-Verden turned from two elective monarchies into a hereditary double monarchy, with a personal rule of the or administrator exchanged for a viceregent government bound by Swedish instructions. The lax administrative structures were replaced with strictly hierarchic authorities with fixed competences. The co-rule of the Estates was curtailed. Bremen and Verden declined from independent territories of imperial immediacy to a collectively governed dominion of a European great power with all the pertaining restrictions and opportunities. For her new fief, the Duchy of Bremen, the Queen regnant Christina of Sweden, ruling from 1644 to 1654, from 1648 on simultaneously Duchess of Bremen and Verden, sought after annexing the Free Imperial City of Bremen for it would be an important taxpayer. Earlier the city of Bremen had de facto participated in the Diets of the Prince-Archbishopric of Bremen. The latter's successor state, the Swedish Duchy of Bremen, tried to regain the city, arguing the Treaty of Westphalia named the city of Bremen as part of the to-be-established Duchy. As Duchess of Bremen and Verden Christina of Sweden installed her residence in the former Benedictine . She abolished witch-burning in Bremen-Verden. In 1650 Charles Gustav, Hereditary Duke of the Palatinate of Zweibrücken-Kleeburg, since 1649 declared and 1650 recognised heir to the Swedish throne and thereby simultaneously to Bremen-Verden's dukedoms, came to Stade for interlocutions of unknown content.