Archbishopric of Magdeburg
The archbishopric of Magdeburg was established as an ecclesiastical principality in 968. In political respect the Erzstift, the archiepiscopal and capitular temporalities, had gained imperial immediacy as prince-archbishopric in 1180. This meant that the archbishop of Magdeburg ruled the town and the lands around it in all matters, worldly and spiritual.
The citizens of Magdeburg had turned Protestant in 1524 and joined the Schmalkaldic League against the religious policies of the Catholic emperor Charles V in 1531. During the Schmalkaldic War of 1546/47, the Lower Saxon city became a refuge for Protestant scholars, which earned it the epithet Herrgotts Kanzlei (German for Lord's Chancellery), but also an Imperial ban that lasted until 1562. The citizens refused to acknowledge Emperor Charles's Augsburg Interim and were besieged by Imperial troops under Maurice, Elector of Saxony in 1550/51.
Protestant archbishops and Administrators
The Roman Catholic archdiocese had de facto turned void since 1557, when the last papally confirmed prince-archbishop, the Lutheran Sigismund of Brandenburg came of age and ascended to the see.
Openly Lutheran Christian William of Brandenburg, elected to be archbishop in 1598, was denied recognition by the imperial authorities. Since about 1600, he styled himself Administrator of Magdeburg, as did other Protestant German notables assigned to govern principalities that were de jure property of the Catholic church.
Alliance with the Danish king
During the Thirty Years' War, Administrator Christian William entered into an alliance with Denmark. In 1626, he led an army from Lower Saxony into the Battle of Dessau Bridge. After Wallenstein won this battle, Christian William fled abroad. In 1629, he fled to the court of King Gustavus Adolphus of Sweden.
As a result of these developments, in January 1628, the Magdeburg cathedral chapter deposed Christian William and elected August of Wettin, 13-year-old son of John George I, Elector of Saxony, as Administrator. This meant little for the moment, however, as Augustus could not assume office due to his father's continued unwillingness to provoke the emperor.
Edict of Restitution
In March 1629, emperor Ferdinand II passed the Edict of Restitution. It was specifically aimed at restoring the situation of the 1555 Peace of Augsburg in ecclesiastical territories that had since strayed from "legal" Catholic faith and rule. Bremen and Magdeburg were the biggest examples of territories to be restituted.
From their viewpoint, the Catholic League army under Tilly were enforcing Imperial law against heretic rebels allying with the enemy, during their 1631 siege of Magdeburg.
Alliance with the Swedish king
The city's councillors had been emboldened by King Gustavus Adolphus's landing in Pomerania on 6 July 1630: the Swedish king was a Lutheran Christian and many of Magdeburg's residents were convinced that he would aid them in their struggle against the Roman Catholic Habsburg emperor, Ferdinand II.
Not all Protestant princes of the Holy Roman Empire had immediately embraced Adolphus, however; some believed his chief motive for entering the war was to take Northern German ports, which would allow him to control commerce in the Baltic Sea. Yet the city of Magdeburg had good reason to ally itself with him: the Swedish army was one of the most efficient of the time and Gustavus Adolphus did not rely on mercenaries as much as other rulers. His army consisted primarily of his countrymen, while the armies of the Holy Roman emperor were a mix of Hungarians, Croats, Spaniards, Poles, Italians, Frenchmen, Germans and others.
In November 1630, King Gustavus sent ex-Administrator Christian William back to Magdeburg, along with Dietrich von Falkenberg to direct the city's military affairs. Backed by the Lutheran clergy, Falkenberg had the suburbs fortified and additional troops recruited.
When the Magdeburg citizens refused to pay a demanded tribute to the emperor, Imperial forces under the command of Johann Tserclaes, Count of Tilly laid siege to the city within a matter of months.
The city was besieged from 20 March 1631 and Tilly put his subordinate Imperial Field Marshal Gottfried Heinrich Graf zu Pappenheim in command while he campaigned elsewhere.
In fierce fighting, the Imperial troops numbering 24,000 conquered several sconces of the city's fortification and Tilly demanded capitulation.