The Thirty Years' War, fought primarily in Central Europe between 1618 and 1648, was one of the most destructive conflicts in European history. An estimated 4.5 to 8 million soldiers and civilians died from the effects of battle, famine, or disease, with parts of Germany reporting population declines of more than 50%. Related conflicts include the Eighty Years' War, the War of the Mantuan Succession, the Franco–Spanish War, the Torstenson War, and Portuguese Restoration War.
Its causes derived from religious conflict within the Holy Roman Empire, sparked by the 16th-century Reformation. The 1555 Peace of Augsburg attempted to resolve this by dividing the empire into Catholic and Lutheran states, but was later destabilised by the expansion of Protestantism beyond these boundaries. Combined with disagreements over the limits of imperial authority, religion was thus an important factor in starting the war. However, its scope and extent were largely the consequence of external drivers such as the French–Habsburg rivalry and the Dutch Revolt.
The war began in 1618, when the Catholic Emperor Ferdinand II was replaced as king of Bohemia by the Protestant Frederick V of the Palatinate. Although Frederick was swiftly deposed, his participation meant fighting expanded into the Palatinate. Their strategic importance drew in the Dutch Republic and the Spanish Empire, then engaged in the Eighty Years' War, while the acquisition of imperial territories gave rulers like Christian IV of Denmark and Gustavus Adolphus of Sweden incentives to intervene. These factors, together with fears the Protestant religion itself was under threat, transformed an internal dynastic dispute into a wider conflict.
The period from 1618 to 1635 was primarily a civil war within the Holy Roman Empire, which largely ended with the Peace of Prague. However, France's entry into the war in alliance with Sweden turned the empire into one theatre of a wider struggle with their Habsburg rivals, Emperor Ferdinand III and Spain. Fighting ended with the 1648 Peace of Westphalia, whose terms included greater autonomy for states like Bavaria and Saxony, Swedish territorial gains in northern Germany, as well as Spain's acceptance of Dutch independence.
Structural origins
The 16th-century Reformation led to the Schmalkaldic War between Protestants and Catholics within the Holy Roman Empire. Fighting ended with the 1552 Peace of Passau, followed by the 1555 Peace of Augsburg, which tried to prevent a recurrence by fixing boundaries between the two faiths. Under the principle of cuius regio, eius religio, states were designated as either Lutheran, then the most usual form of Protestantism, or Catholic, based on the religion of their ruler. Other provisions protected substantial religious minorities in cities like Donauwörth, and confirmed Lutheran ownership of property taken from the Catholic Church since 1552.
However, the settlement was undermined by the expansion of Protestantism into Catholic areas post-1555, particularly Calvinism, a Protestant doctrine viewed with hostility by both Lutherans and Catholics. It also gave individual states greater freedom to pursue their own objectives, which often clashed with those of central authority or local rivals. Disputes over money and territory superseded religion, with the Protestant states of Saxony and Brandenburg competing with Denmark–Norway and Sweden for the lucrative Baltic trade.
Managing tensions was hampered by fragmented political institutions, with 300 imperial estates distributed across Germany, the Low Countries, northern Italy, and France. These ranged in size and importance from the seven Prince-electors who voted for the Holy Roman Emperor, down to Prince-bishoprics and imperial cities like Hamburg. Each also belonged to a separate regional grouping known as an Imperial Circle, which was chiefly concerned with defence, and operated independently. Above all of these was the Imperial Diet, which assembled infrequently, and focused on discussion, rather than legislation.
Since 1440, the position of emperor had been held by the House of Habsburg, the largest single landowner within the Holy Roman Empire. They directly ruled over eight million subjects and territories that included Austria, Bohemia and Hungary. The Habsburgs also controlled the Spanish Empire until 1556, when Charles V divided his possessions between different branches of the family. This bond was reinforced by frequent inter-marriage, while Spain retained territories within the Holy Roman Empire such as the Spanish Netherlands, Milan and Franche-Comté.
Though the Spanish and Austrian branches often collaborated, they did not generally coordinate their policies because the two entities were very different. Spain was a global maritime superpower, stretching from Europe to the Philippines and the Americas, while Austria was a land-based power, focused on Germany and securing its eastern border against the Ottoman Empire. Another key difference was the disparity in relative financial strength, with the Spanish Habsburgs providing large subsidies to their Austrian counterparts. The loss of these post-1640, as Spain itself struggled with the costs of a long running global war, substantially weakened the imperial position.
Prior to the Reformation, shared religion partially compensated for weak imperial institutions. After 1556, rising religious and political tensions allowed states like Lutheran Saxony and Catholic Bavaria to expand their own power, while further weakening imperial authority. This internal political struggle was exacerbated by external powers with their own strategic objectives, such as Spain, the Dutch Republic, or France, confronted by Habsburg lands on its borders to the north, south, and along the Pyrenees. Since a number of foreign rulers were also imperial princes, divisions within the empire drew in players like Christian IV of Denmark, who joined the war in 1625 as Duke of Holstein-Gottorp.
Background: 1556 to 1618
Disputes occasionally resulted in full-scale conflict like the 1583 to 1588 Cologne War, triggered when its ruler converted to Calvinism. More common were localised riots such as the 1606 Battle of the Flags in Donauwörth, caused when the Lutheran majority blocked a Catholic religious procession. Emperor Rudolf allowed Maximilian of Bavaria, a Catholic, to annex the town, changing the official religion as agreed at Augsburg.
When the Imperial Diet opened in February 1608, both Lutherans and Calvinists sought re-confirmation of the Augsburg settlement. In return, the Habsburg heir Archduke Ferdinand required the immediate restoration of any property taken from the Catholic Church since 1555, as opposed to the previous practice whereby each case was assessed separately. By threatening all Protestants, his demand paralysed the Diet, and removed the perception of imperial neutrality.
Many towns and rulers responded by strengthening their military defences, as noted by foreign travellers. When Frederick IV, Elector Palatine formed the Protestant Union in 1608, Maximilian established the rival Catholic League in July 1609. Both were largely vehicles for their leaders' dynastic ambitions, but combined with the 1609 to 1614 War of the Jülich Succession, tensions increased throughout the empire.
What started as an internal dynastic dispute drew in external powers due to the imminent expiry of the 1609 Twelve Years' Truce, which suspended the Eighty Years' War between Spain and the Dutch Republic. Before restarting hostilities, Ambrosio Spinola, commander in the Spanish Netherlands, needed to secure the Spanish Road, an overland route connecting Habsburg possessions in Italy to Flanders. This allowed them to transport troops and supplies by road rather than sea, which was dominated by the Dutch navy. By 1618, the only part not controlled by Spain ran through the Electoral Palatinate.
Since Emperor Matthias had no surviving children, in July 1617 Philip III of Spain agreed to support Ferdinand II's election as king of Bohemia and Hungary. In return, Ferdinand made concessions to Spain in northern Italy and Alsace and agreed to support their offensive against the Dutch. Doing so required his election as emperor, which was not guaranteed; Maximilian of Bavaria, who opposed the increase of Spanish influence in an area he considered his own, tried to create a coalition to support his candidacy.
Another option was Frederick V, Elector Palatine, a Calvinist who in 1613 married Elizabeth of Bohemia, daughter of James I of England. When Ferdinand was elected king of Bohemia in 1617, he also gained control of its electoral vote, but his conservative Catholicism made him unpopular with the mostly Protestant Bohemian nobility, who were also concerned about the erosion of their rights. Since the seven electors previously consisted of three Protestants and four Catholics, replacing Ferdinand with Frederick would alter the religious balance, making possible the election of a Protestant emperor and the end of Habsburg predominance. These factors combined to bring about the Bohemian Revolt in May 1618.
Phase I: 1618 to 1635
Bohemian Revolt
Ferdinand once claimed he would rather see his lands destroyed than tolerate heresy within them. Less than 18 months after taking control of Styria in 1595, he had eliminated Protestantism in what had been a stronghold of the Reformation. Their war in the Netherlands meant the Spanish Habsburgs preferred to avoid antagonising Protestants elsewhere. However, while recognising the dangers of Ferdinand's fervent Catholicism, they ultimately supported his claim due to lack of alternatives.
Elected king of Bohemia in May 1617, Ferdinand reconfirmed Protestant religious freedoms, but his record in Styria led to the suspicion he was only awaiting a chance to overturn them. These concerns were heightened after a series of legal disputes over property were all decided in favour of the Catholic Church. In May 1618, Protestant nobles led by Count Thurn met in Prague Castle with Ferdinand's two Catholic representatives, Vilem Slavata and Jaroslav Borzita. In what became known as the Third Defenestration of Prague, both men were thrown out of the castle windows along with their secretary Filip Fabricius, although all three survived.
Thurn established a Protestant-dominated government in Bohemia, while unrest expanded into Silesia and the Habsburg heartlands of Lower and Upper Austria, where much of the nobility was also Protestant. Losing control of these threatened the entire Habsburg state, while in addition to its crucial electoral vote, Bohemia was one of the most prosperous areas of the Empire. Regaining control was vital for the Austrian Habsburgs, but chronic financial weakness left them dependent on Maximilian and Spain for the resources needed to achieve this.
Spanish involvement in Bohemia drew in the Dutch and their ally, France, although the strongly Catholic Louis XIII faced his own domestic Protestant rebellion and refused to support them elsewhere. Elsewhere, Charles Emmanuel I, Duke of Savoy helped Frederick fund a mercenary army under Ernst von Mansfeld, which was sent to support the Bohemian rebels. Attempts by Maximilian and John George of Saxony to broker a negotiated solution ended when Emperor Matthias died in March 1619, since many believed the loss of his authority and influence had fatally damaged the Habsburgs.
By mid-June 1619, an army led by Thurn was outside Vienna, and despite defeating Mansfeld at Sablat, Ferdinand's position continued to worsen. Gabriel Bethlen, the Calvinist Prince of Transylvania, invaded Hungary and besieged Vienna with Ottoman support, although war with Poland–Lithuania in 1620, followed by the 1623 to 1639 conflict with Persia, prevented their direct participation.
On 19 August, the Bohemian Estates rescinded Ferdinand's 1617 election as king and a week later formally offered the crown to Frederick. On 28 August, Ferdinand was elected Holy Roman Emperor, making war inevitable if Frederick accepted the Bohemian crown. Most of Frederick's advisors urged him to reject it, as did the Duke of Savoy, and his father-in-law James I.
However, he received backing from Christian of Anhalt, along with Maurice of Orange, for whom conflict in Germany was a means to divert Spanish resources from the Netherlands. The Dutch offered subsidies to Frederick and the Protestant Union, helped raise loans for Bohemia, and provided weapons and munitions. However, wider European support failed to materialise, largely due to lack of enthusiasm for removing a legally elected ruler, regardless of religion.
Although Frederick accepted the crown and entered Prague in October 1619, his support eroded over the next few months. In July 1620, the Protestant Union proclaimed its neutrality, while John George of Saxony backed Ferdinand in return for the cession of Lusatia and a guarantee of Lutheran rights in Bohemia. Maximilian of Bavaria funded a combined Imperial-Catholic League army led by Count Tilly and the Count de Bucquoy, which pacified Upper and Lower Austria and occupied western Bohemia before marching on Prague. Defeated at the Battle of White Mountain in November 1620, the Bohemian army disintegrated, and Frederick fled the country.
Palatinate campaign
By abandoning Frederick, the German princes hoped to restrict the dispute to Bohemia, an objective thwarted by Maximilian's dynastic ambitions. In the October 1619 Treaty of Munich, Ferdinand transferred the Palatinate's electoral vote to Bavaria, and allowed Maximilian to annex the Upper Palatinate. Many Protestants had supported Ferdinand against Frederick because they opposed the deposition of a legally elected ruler, and now objected to his removal on the same grounds. For Catholics, it presented an opportunity to regain lands and properties lost since 1555, a combination which destabilised large parts of the Empire.