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.
Citation:
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 22 May 2013.]