[1][2] Despite its status as a free imperial city, Aachen was under the protection of John Sigismund of Brandenburg, Neuburg's ally, and then rival, in the battle for the United Duchies of Jülich-Cleves-Berg.

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.

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.

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.

[6] 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.

[9] 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.

[10] 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.

[16] After several days of negotiations, and with little hope of reinforcements, the defenders surrendered the city to the Spanish army, to the great regret of Maurice, who could not arrive in time for relief.

[18] The Catholic city council was restored, and, on 10 September, it issued an edict which gave the Protestant preachers three days to abandon the town, and six weeks for the non-citizen Anabaptists and other foreigners to do the same.

[19] From then on, only Catholic schools and schoolmasters were tolerated, books labeled as heretics were banned, meat dishes were not allowed to be eaten in inns on the fast days, and a fitting homage was to be paid to the Holy Sacrament and relics when public processions were held.

[19] After the capture of Aachen, Spinola took several towns and castles in the lands disputed by the claimants to the Jülich heritage, including Neuss, Mülheim, and the important German fortress-city of Wesel, whish was garrisoned by troops of Brandenburg,[20] As a consequence, the Protestant position in the Rhineland was weakened despite Spinola deciding not to lay siege to Jülich due to the city's strong defense and large garrison.