The Thirty Years’ War was a 17th-century religious conflict fought primarily in central Europe. It remains one of the longest and most brutal wars in human history, with more than 8 million casualties resulting from military battles as well as from the famine and disease caused by the conflict. The war lasted from 1618 to 1648, starting as a battle among the Catholic and Protestant states that formed the Holy Roman Empire. However, as the Thirty Years’ War evolved, it became less about religion and more about which group would ultimately govern Europe. In the end, the conflict changed the geopolitical face of Europe and the role of religion and nation-states in society.

Causes of the Thirty Years’ War

With Emperor Ferdinand II’s ascension to head of state of the Holy Roman Empire in 1619, religious conflict began to foment.

One of Ferdinand II’s first actions was to force citizens of the empire to adhere to Roman Catholicism, even though religious freedom had been granted as part of the Peace of Augsburg.

Signed in 1555 as a keystone of the Reformation, the Peace of Augsburg’s key tenet was “whose realm, his religion,” which allowed the princes of states within the realm to adopt either Lutheranism/Calvinism or Catholicism within their respective domains.

This effectively calmed simmering tensions between peoples of the two faiths within the Holy Roman Empire for more than 60 years, although there were flare ups, including the Cologne War (1583-1588) and the War of the Julich Succession (1609).

Still, the Holy Roman Empire may have controlled much of Europe at the time, though it was essentially a collection of semi-autonomous states or fiefdoms. The emperor, from the House of Habsburg, had limited authority over their governance.

Defenestration of Prague

But after Ferdinand’s decree on religion, the Bohemian nobility in present-day Austria and the Czech Republic rejected Ferdinand II and showed their displeasure by throwing his representatives out of a window at Prague Castle in 1618.

The so-called Defenestration of Prague (fenestration: the windows and doors in a building) was the beginning of open revolt in the Bohemian states – who had the backing of Sweden and Denmark-Norway – and the beginning of the Thirty Years’ War.

Bohemian Revolt

In response to Ferdinand II’s decision to take away their religious freedom, the primarily Protestant northern Bohemian states of the Holy Roman Empire sought to break away, further fragmenting an already loosely structured realm.

The first stage of the Thirty Years’ War, the so-called Bohemian Revolt, began in 1618 and marked the beginning of a truly continental conflict. Over the first decade-plus of fighting, the Bohemian nobility formed alliances with the Protestant Union states in what is now Germany, while Ferdinand II sought the support of his Catholic nephew, King Phillip IV of Spain.

Soon, armies for both sides were engaged in brutal warfare on multiple fronts, in present-day Austria and in the east in Transylvania, where Ottoman Empire soldiers fought alongside the Bohemians (in exchange for yearly dues paid to the sultan) against the Poles, who were on the side of the Habsburgs.

Catholic League Victories

To the west, the Spanish army aligned with the so-called Catholic League, nation-states in present-day Germany, Belgium and France, who supported Ferdinand II.

At least initially, Ferdinand II’s forces were successful, quelling the rebellion to the east and in northern Austria, leading to the dissolution of the Protestant Union. However, fighting continued to the west, where Denmark-Norway’s King Christian IV threw his support behind the Protestant states.

Even with help from soldiers from Scotland, however, the armies of Denmark-Norway fell to the forces of Ferdinand II, ceding much of northern Europe to the emperor.

Gustavus Adolphus

But in 1630, Sweden, under the leadership of Gustavus Adolphus, took the side of the northern Protestants and joined the fight, with its army helping to push Catholic forces back and regain much of the lost territory lost by the Protestant Union.

With the support of the Swedes, Protestant victories continued. However, when Gustavus Adolphus was killed in the Battle of Lutzen in 1632, the Swedes lost some of their resolve.

Using military assistance of Bohemian nobleman Albrecht von Wallenstein, who provided his army of an estimated 50,000 soldiers to Ferdinand II in exchange for the freedom to plunder any captured territory, began to respond and, by 1635, the Swedes were vanquished.

The resulting treaty, the so-called Peace of Prague, protected the territories of the Lutheran/Calvinist rulers of northeastern Germany, but not those of the south and west in present-day Austria and the Czech Republic. With religious and political tensions in the latter regions remaining high, fighting continued.

French Involvement

The French, though Catholic, were rivals of the Habsburgs and were unhappy with the provisions of the Peace of Prague.

Thus, the French entered the conflict in 1635. However, at least initially, their armies were unable to make inroads against the forces of Ferdinand II, even after he died of old age in 1637.

Meanwhile, Spain, fighting at the behest of the emperor’s successor and son, Ferdinand III, and later under Leopold I, mounted counter-attacks and invaded French territory, threatening Paris in 1636. However, the French recovered, and fighting between the French-Protestant alliance and the forces of Spain and the Holy Roman Empire were at a stalemate for the next several years.

In 1640, the Portuguese began to revolt against their Spanish rulers, thereby weakening their military efforts on behalf of the Holy Roman Empire. Two years later, the Swedes re-entered the fray, further weakening Habsburg forces.

A Shift in the Thirty Years’ War

The next year, 1643, was pivotal in the decades-long conflict. That year, Denmark-Norway took up arms again, this time fighting on the side of the Habsburgs and the Holy Roman Empire.

At around the same time, French monarch Louis XIII died, leaving the throne to his 5-year-old son, Louis XIV, and creating a leadership vacuum in Paris.

Over the ensuing years, the French army had several notable victories, but also suffered significant defeats, particularly at the Battle of Herbsthausen in 1645. Also in 1645, the Swedes attacked Vienna, but were unable to capture the city from the Holy Roman Empire.

Prague Castle Captured

In 1647, the Habsburg forces led by Octavio Piccolomini were able to repel the Swedes and the French from what is now Austria.

The next year, in the Battle of Prague – the last significant fighting in the Thirty Years’ War – the Swedes captured Prague Castle from the forces of the Holy Roman Empire (and looted the priceless art collection in the castle), but were unable to take the bulk of the city.

By this time, only the Austrian territories remained under the control of the Habsburgs.

Peace of Westphalia

Over the course of 1648, the various parties in the conflict signed a series of treaties called the Peace of Westphalia, effectively ending the Thirty Years’ War – although not without significant geopolitical effects for Europe.

Weakened by the fighting, for example, Spain lost its grip over Portugal and the Dutch republic. The peace accords also granted increased autonomy to the former Holy Roman Empire states in German-speaking central Europe.

Legacy of the Thirty Years’ War

Ultimately, though, historians believe the Peace of Westphalia laid the groundwork for the formation of the modern nation-state, establishing fixed boundaries for the countries involved in the fighting and effectively decreeing that residents of a state were subject to the laws of that state and not to those of any other institution, secular or religious.

This radically altered the balance of power in Europe and resulted in reduced influence over political affairs for the Catholic Church, as well as other religious groups.

As brutal as the fighting was in the Thirty Years’ War, hundreds of thousands died as a result of famine caused by the conflict as well as an epidemic of typhus, a disease that spread rapidly in areas particularly torn apart by the violence. Historians also believe the first European witch hunts began during the war, as a suspicious populace attributed the suffering throughout Europe at the time to “spiritual” causes.

The war also fostered a fear of the “other” in communities across the European continent, and caused an increased distrust among those of different ethnicities and religious faiths – sentiments that persist to some degree to this day.


“The Economist explains: What happened in the Thirty Years War?”

Catholic Encyclopedia. “The Thirty Years War.”

Sommerville, J.P. “The aftermath of the Thirty Years’ War.”