In the late 1630s, Europe was on the brink of a major conflict, largely driven by religious tensions between Catholic and Protestant states within the Holy Roman Empire (HRE) and Central Europe. Religious oppression was common, with few states tolerating minorities, leading to devastating consequences for social cohesion and conformity. The Habsburgs, preoccupied with maintaining influence in the Balkans against Eastern Rome, managing the Empire in Germany, and dealing with the Muslim state in Southern Italy, had little patience for what they viewed as the "Protestant nuisance".

The conflict formally began on February 10th, 1641, in Luben, when imperial forces were sent into the city to enforce tax collection. This action escalated into a riot and then an open revolt by the Protestant Silesians. Determined to resist religious oppression, Silesia sought assistance from the Protestant states of the empire.

The Protestant states responded on May 2nd, 1641, when Brandenburg moved directly into Silesia, claiming to protect Protestant rights from Catholic encroachment. To the Habsburgs, this was a step too far, necessitating a response. On July 6th, 1641, near Breslau, the Austrian and Brandenburger armies clashed. Although the Brandenburgers initiated an attack during negotiations, the battle's outcome was inconclusive; regardless, Austria and Brandenburg were officially at war.

Both sides quickly rallied support. Bavaria and the Catholic HRE states backed the Habsburgs, while the Northern HRE states supported Brandenburg. Initial skirmishes between late 1641 and mid-1642 saw both sides avoiding direct engagement until they felt positioned for strength. This period ended on June 10th, 1642, near Jena, where Anti-Brandenburg forces surprised the Pro-Brandenburg forces, resulting in a marginal Anti-Brandenburg victory and the destruction of Jena itself.

Later that year, France joined the war on the side of the Habsburgs. France viewed Protestant expansion as a more urgent threat than their disputes with the Habsburgs. French calculus suggested that a strong, Catholic Habsburg empire, free from "petty religious squabbles," would be better positioned to deal with the Muslims to France's south, serving French interests.

From 1642 to 1645, the Pro-Brandenburg alliance fought a defensive and protracted war against the Anti-Brandenburg armies. Northern Germany endured savage clashes, widespread devastation, pillages, and reprisals.

In November 1645, Denmark entered the war on the side of the Pro-Brandenburg alliance, motivated by a desire to resist what they called the "Catholic Terror". However, this boost in morale was quickly negated by the Battle of Waren on February 20th, 1646, where Danish forces were defeated by French and Austrian armies and forced into a rout due to icy conditions.

Despite the dominance of the Anti-Brandenburg alliance at this point, the situation changed later in 1646. The Swedish-Polish Union, long allied with Austria, launched a simultaneous invasion of Denmark and Brandenburg in August 1646. The Pro-Brandenburg alliance attempted to preempt the Union's heavy toll by engaging them directly. On October 5th, 1646, near Luckau, a combined Danish-Brandenburger force attacked the Polish-Swedish forces. The Danish-Brandenburger army was caught by surprise when the Polish-Swedish cavalry, the formidable 'Winged Hussars', executed a delayed flank attack, leading to a swift rout. The Battle of Luckau was devastating for the Pro-Brandenburg cause, which lost thousands of men.

By the start of 1648, the Pro-Brandenburg alliance faced dire circumstances: Northern Germany was ravaged, large parts of Denmark and Brandenburg were occupied, and they were opposed by the combined might of France, the Habsburgs, and the Swedish-Polish Union.

However, the tide turned when the Orthodox empires intervened. Russia and Eastern Rome, both having scores to settle with the Swedish-Poles and Habsburgs respectively, viewed the conflict as an opportunity. In February 1648, Russia and Eastern Rome launched invasions of Sweden-Poland and Bohemia, catching the Anti-Brandenburg alliance completely off guard. The Russians quickly occupied the Swedish-Polish hinterlands, while the Eastern Romans engaged in brutal battles across the Bohemian plains.

The war culminated in the Battle of Schwandorf on November 3rd, 1650, a multi-day bloodbath that utterly destroyed the town. Two large armies clashed: 40,000 Anti-Brandenburg forces (Austrian and French) against 35,000 Pro-Brandenburg forces (Brandenburger, Russian, and Eastern Roman). The Pro-Brandenburg side eventually won, though the victory was pyrrhic.

By August 1651, all parties agreed that the war had moved beyond their initial objectives, and continuing the conflict would only cause more death and destruction. The ensuing peace treaty, stipulated at The Hague, largely restored the status quo but included guarantees for religious freedoms and the curtailment of religious oppression within the HRE.

The Kingdom of Sweden benefited significantly from the conflict. Despite the overall failure of the war to achieve decisive goals, Sweden was able to secure territories in northern Germany and Danish Holstein, which their forces had occupied, in the peace treaty. As a result, Sweden, and the Polish-Swedish Union as a whole, effectively turned the Baltic Sea into their "own personal lake".

The Ten Years' War proved to be one of the most brutal Europe had ever seen, ultimately concluding with no faction achieving its primary goals and seeming like a waste of blood and time.