The siege of Aachen took place in 24 August 1614, when the Spanish Army of Flanders, led by Ambrogio Spinola, 1st Marquis of the Balbases, marched from Maastricht to Germany to support Wolfgang Wilhelm, Count Palatine of Neuburg, during the War of the Jülich Succession. Despite its status as a free imperial city, Aachen was under the protection of John Sigismund of Brandenburg, Neunburg's ally, and then rival, in the battle for the United Duchies of Jülich-Cleves-Berg. In 1611, the Protestant population of Aachen had revolted against the Catholic city council and had seized power. When the Holy Roman Emperor Rudolf II, observing the Peace of Augsburg, had ordered the previous state to be restored, the Protestants had allied themselves with the Margraviate of Brandenburg. The unexpected arrival of a Spanish army at the gates of the city, however, caused the Protestants to lose courage and surrender Aachen to Spinola. A Catholic garrison was installed and a process of re-Catholicization began.
Background
Aachen was a free imperial city of importance from the times of Charlemagne to the 16th century. It was the place of coronation of the King of the Germans until Maximilian II was crowned in Frankfurt in 1562. Since then, Aachen went into a slow decline. A mainly Catholic city at the time of the Peace of Augsburg, it became religiously divided in the 1560s through the immigration of Protestant refugees from the Netherlands as a consequence of the Spanish persecution during the Dutch revolt. By the 1570s Aachen's population numbered 12,000 Catholics and 8,000 Protestants. The city council and the Emperor tried to exclude the Protestants from political participation in 1581, but thanks to the economic influence of many of the Protestant citizens, Catholics were forced to allow them access to the city council. As jurisdiction over Aachen was claimed by the Duke of Jülich and the Bishop of Liege, both of them Catholics, the Catholic population appealed to the first, who complained to Emperor Rudolf II, claiming that his ecclesiastic rights over Aachen had been violated. In 1593 the Reichshofrat declared that the city council could not change the religious status of Aachen, and therefore the Calvinists had to be expelled from the council. When they tried to resist, Rudolf outlawed the city and gave Archduke Albert, governor of the Spanish Netherlands, the task of enforcing his decision. The subsequent re-Catholicization of the city was conducted by the Archbishop of Cologne.
In 1611, during the War of the Jülich Succession, the Elector of Brandenburg and the Duke of Palatinate-Neuburg, claimants of the Jülich heirdom in opposition to the Catholic Leopold V, Archduke of Austria, organized Protestant religious services in the nearby villages of Stolberg and Weiden. In response, the city council of Aachen imposed a fine on those inhabitants who attended these services. Five citizens were detained for ignoring the town's decree and banished as they refused to pay the fine. This caused a riot against the council on 5 July. The Catholic counsellors were expelled and many Catholic buildings were sacked. The rebels assaulted the church and the college of the Jesuits, smashed the altars and images, and held a mock mass dressed in priestly garments. One priest was injured and eight others dragged to the city council. A new Protestant council was established and appealed for support to John Sigismund, Elector of Brandenburg, the new Duke of Cleves-Mark.
Spanish intervention
Rudolf II ordered the princes of Brandenburg and Palatinate-Neuburg to restore the previous religious and political situation of Aachen under the menace of a ban. The Protestants, however, ignored the command and seriously wounded an Imperial commissary sent to implement the Emperor's edict. In May 1612, elections were held and Calvinists took control of the town council. In 1613, as disputes over the Jülich succession continued, one of the claimants, Wolfgang Wilhelm, Count Palatine of Neuburg, converted to Catholicism and gained the support of Spain and of the Catholic League of Germany. On 20 February 1614, Emperor Matthias ordered the restoration of Catholic rule in Aachen, allowing the Spanish Army of Flanders under Ambrogio Spinola to intervene. Fearing an attack, the town council requested help from the Elector of Brandenburg, who sent several hundred soldiers under general Georg von Pulitz to reinforce the local militia. The city gates were manned and partly walled-up.