Magdeburg had adopted the Reformation in the 1520s, and taken part in the Schmalkaldic League. After the League's defeat in the war against the Emperor of 1546-7, it refused to accept the Augsburg Interim and was put under the ban of the Empire. Charles V sent Duke Maurice of Saxony to besiege the city, countered by troops from the Dukes of Prussia and Mecklenburg, but the citizens refused to submit until November 1551.
Editors. "Magdeburg besieged by Maurice of Saxony". The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=10074, accessed 25 January 2015.]