At first, the war pitted Catholic Archduke Leopold V against the combined forces of the Protestant claimants, Johann Sigismund, Elector of Brandenburg and Wolfgang Wilhelm of Palatinate-Neuburg, ending in the former's military defeat.

By the early 17th century, the rapid spread of the Lutheran and Calvinist doctrines after the Protestant Reformation was met by a period of Roman Catholic resurgence, known as the Counter-Reformation.

The territories in question covered an area of 14,000 km² and had both geopolitical importance from their proximity to the Spanish Road and a booming economy, which was fueled by refugees fleeing the lands that were being ravaged by the Eighty Years' War.

The immediate cause of war was the death of John William, Duke of Jülich-Cleves-Berg on 25 March 1609, who was affected by serious mental illness, and left no heirs to succeed him.

A total of six other claimants appeared, with rulers of the Margraviate of Brandenburg and Palatinate-Neuburg having the only credible claims through their relationship to John William's sisters.

As Neuburg had benefited from aligning with the Emperor in the past, despite their Lutheran religious status, Wolfgang Wilhelm had personally travelled to Prague to gain Imperial support.

On 24 May, Rudolf II, who had the authority to dictate a final settlement over the succession, announced that the Aulic Council (one of the two Supreme Courts in the Holy Roman Empire) was to provide a definite verdict within four weeks.

[1] Brandenburg and Neuburg, who viewed the regency as a direct attempt at undermining their claims, finally agreed to a provisional settlement, with mediation from Maurice of Hesse-Kassel.

[1] When Joachim Ernst, a representative of Johann Sigismund, passed by Kassel on his way to Jülich, Maurice convinced him to negotiate with Wolfgang Wilhelm.

[1] Though initially hesitant and urged by Imperial agents to wait for a decision from Emperor Rudolf, by 22 July, most of the estates had sworn fealty to their new rulers.

[2] Emperor Rudolf hurriedly intervened while the Protestant claimants of Neuburg and Brandenburg, also known as the possessors, (German: possidierenden)[1][Footnotes 1] had still not fully secured the duchies.

He demanded the annulment of the Dortmund treaty and sent another Imperial Commissioner, Archduke Leopold V, Bishop of Passau and Strasbourg, to take charge.

"[7] Word reached Paris on 29 July, and Henry IV called his chief advisors (Villeroy, Sully, Sillery, and Jeannin) for advise on what to do.

He believed that not helping the Protestants would harm France's international reputation, and that letting the Habsburgs dominate in Jülich would worsen the Dutch Republic's position (a major French ally at this time).

In the meantime, Henry called for the raising of levies in northeastern France and worked to move French regiments in the Netherlands to the borders Jülich.

John Frederick supported his claim by stating that: Wolfgang Wilhelm and Ernst also failed to cooperate with each other, even after the signing of the Dortmund Treaty.

First, the duchies of Jülich-Cleves-Berg lay along the Rhine, and allowing it to fall to the Spanish or some other Habsburg sympathizer could be politically and economically crippling for the Protestant princes.

[5] Though the coronation of the young Louis XIII happened smoothly, there was much that Marie de' Medici, Henry's wife and now regent of France, needed to do to stabilize the country.

This force was to be transported by land to Jülich,[Footnotes 7] led by Marshall Claude de la Châtre,[9] and began mobilizing at Metz on 5 July.

[1] In the meantime, the Protestants succeeded in seizing Dachstein, Mutzig and Molsheim, but these advances were brought to halt after the local population had refused to supply it food.

[Footnotes 8] Nonetheless, the pro-War party won out, and la Châtre was given orders to march on Jülich, but due to Condé, his expedition was delayed until late July.

Moreso, the Spanish Council decided that instead of acting neutral in the case of French intervention, Spain should oppose the possessors to counter the Dutch.

Regardless, when Spain discovered that France maintained their agreement to send a small army to aid the possessors, they decided to remain out of the war.

The rebels assaulted the church and the College of the Jesuits, smashed the altars and images and held a mock mass by being dressed in confiscated priestly garments.

Archduke Albert stated that he would intervene on the side of whoever was attacked, but in reality he was anxious to avoid any war, despite the Spanish government preferring Neuburg.

However, his commitment appeared to wane over time, with the English position shifting to assert that England and the Dutch should settle the matter without interference from the Spanish Netherlands.

Additionally, some parties, including the Dutch envoy in Paris and Wolfgang Wilhelm himself, expressed skepticism about the feasibility and effectiveness of such a plan.

Spain gained a total of 62 towns including three crossings of the Rhine (Wesel, Orsoy and Rheinberg), which significantly enhanced its position in North–western Europe.

The Dutch retained their garrisons at Jülich and Pfaffenmütze but were now outflanked or even isolated, which put them in an unfavorable position when the Twelve Years' Truce expired in April 1621.

The towns in Cleves were protected by the Dutch, but the rest of the territory was raided and burdened with high contributions and stationing, due to the movements of Swedish, Imperial, and Hessian soldiers.