The 

Thirty Years' War[l] was a conflict fought largely within the Holy Roman Empire from 1618 to
1648. Considered one of the most destructive wars in European history, estimates of total deaths
caused by the conflict range from 4.5 to 8 million, while some areas of Germany experienced
population declines of over 50%.[18] Related conflicts include the Eighty Years' War, the War of
the Mantuan Succession, the Franco-Spanish War, and the Portuguese Restoration War.
Until the 20th century, historians considered it a continuation of the German religious
struggle initiated by the Reformation and ended by the 1555 Peace of Augsburg. This divided the
Empire into Lutheran and Catholic states, but over the next 50 years the expansion of
Protestantism beyond these boundaries gradually destabilised Imperial authority. While religion
was a significant factor in starting the war that followed, it is generally agreed that its scope and
extent was driven by the contest for European dominance
between Habsburgs in Austria and Spain, and the French House of Bourbon.[19]
The war began in 1618 when Ferdinand II was deposed as King of Bohemia and replaced
by Frederick V of the Palatinate. Although the Bohemian Revolt was quickly suppressed, fighting
expanded into the Palatinate, whose strategic importance drew in the Dutch Republic and Spain,
then engaged in the Eighty Years War. Since ambitious external rulers like Christian IV of
Denmark and Gustavus Adolphus of Sweden also held territories within the Empire, what began
as an internal dynastic dispute was transformed into a far more destructive European conflict.
The first phase from 1618 until 1635 was primarily a civil war between German members of the
Holy Roman Empire, with external powers playing supportive roles. After 1635, the Empire
became one theatre in a wider struggle between France, supported by Sweden, and Spain in
alliance with Emperor Ferdinand III. This concluded with the 1648 Peace of Westphalia, whose
provisions included greater autonomy within the Empire for states like Bavaria and Saxony, as
well as acceptance of Dutch independence by Spain. By weakening the Habsburgs relative to
France, the conflict altered the European balance of power and set the stage for the wars
of Louis XIV.

Structural origins[edit]
The 1552 Peace of Passau ended the Schmalkaldic War between Protestants and Catholics in
the Holy Roman Empire, while the 1555 Peace of Augsburg tried to prevent future conflict by
fixing existing boundaries. Under the principle of cuius regio, eius religio, states were
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 Passau.[20]
The agreement was undermined by the expansion of Protestantism beyond its 1555 boundaries,
into areas previously dominated by Catholicism. An additional source of conflict was the growth
of Reformed faiths not recognised by Augsburg, especially Calvinism, a theology viewed with
hostility by both Lutherans and Catholics.[21] Finally, religion was increasingly superseded by
economic and political objectives; Lutheran Saxony, Denmark-Norway and Sweden competed
with each other and Calvinist Brandenburg over the Baltic trade.[22]
Managing these issues was complicated by the fragmented nature of the Empire, which had
nearly 1,800 separate entities distributed across Germany, the Low Countries, Northern Italy, as
well as Alsace and Franche-Comté in modern France. They 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.[m] Each member belonged to a regional assembly
or Circle, which focused on defence and taxes and often operated as autonomous bodies. Above
these structures was the Imperial Diet, which prior to 1663 assembled on an irregular basis, and
served primarily as a forum for discussion, rather than legislation.[23]
Although Emperors were elected, since 1440 the position had been held by a member of
the Habsburg family. The largest single landowner within the Empire, they controlled territories
containing over eight million subjects, including the Archduchy of Austria, Bohemia and Hungary.
The Habsburg emperors also ruled Spain until 1556, when it became a separate entity; Spain
retained Imperial interests, including the Duchy of Milan, and while the two branches of the family
often co-operated, their objectives did not always align. The Spanish Empire was a global
maritime superpower whose possessions included the Spanish Netherlands, Milan, the Kingdom
of Naples, the Philippines, and most of the Americas. Austria was a land-based power, whose
strategic focus was securing a pre-eminent position in Germany and their eastern border against
the Ottoman Empire.[24]
Before Augsburg, unity of religion compensated for lack of strong central authority; once
removed, it presented opportunities for those who sought to further weaken it. These included
ambitious Imperial states like Lutheran Saxony and Catholic Bavaria, as well as France,
confronted by Habsburg lands on its borders to the North, South, and along the Pyrenees. A
further complication was that many foreign rulers were also Imperial princes, involving them in its
internal disputes; Christian IV of Denmark joined the war in 1625 as Duke of Holstein.[25]
Disputes occasionally resulted in full-scale conflict like the 1583 to 1588 Cologne War, caused
when its ruler converted to Calvinism. More common were events such as the 1606 'Battle of the
Flags' in Donauwörth, when riots broke out after the Lutheran majority blocked a Catholic
religious procession. Emperor Rudolf approved intervention by the Catholic Maximilian of
Bavaria. In return, he was allowed to annex the town and as agreed at Augsburg, the official
religion changed from Lutheran to Catholic.[26]
When the Imperial Diet opened in February 1608, both Lutherans and Calvinists united to
demand formal re-confirmation of the Augsburg settlement. However, in return the Habsburg
heir Archduke Ferdinand required the immediate restoration of all property taken from the
Catholic church since 1555, rather than the previous practice whereby court ruling case by case.
This threatened all Protestants, paralysed the Diet, and removed the perception of Imperial
neutrality.[27]
Loss of faith in central authority meant towns and rulers began strengthening their fortifications
and armies; outside travellers often commented on the growing militarisation of Germany in this
period.[28] This was taken a stage further in 1608 when Frederick IV, Elector Palatine formed
the Protestant Union and Maximilian responded by setting up the Catholic League in July 1609.
Both Leagues were primarily designed to support the dynastic ambitions of their leaders, but their
creation combined with the 1609 to 1614 War of the Jülich Succession to increase tensions
throughout the Empire.[29] Some historians who see the war as primarily a European conflict
argue Jülich marks its beginning, with Spain and Austria backing the Catholic candidate, France
and the Dutch Republic the Protestant.[30]
External powers became involved in what was an internal German dispute 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 him to move troops and supplies by
road, rather than sea where the Dutch navy was dominant; by 1618, the only part not controlled
by Spain ran through the Electoral Palatinate.[31]
Since Emperor Matthias had no surviving children, in July 1617 Philip III of Spain agreed to
support Ferdinand'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. Delivering these commitments required his election as Emperor, which was not
guaranteed; one alternative was Maximilian of Bavaria, who opposed the increase of Spanish
influence in an area he considered his own, and tried to create a coalition with Saxony and the
Palatinate to support his candidacy.[32]
A third candidate was the Calvinist Frederick V, Elector Palatine, who succeeded his father in
1610, then in 1613 married Elizabeth Stuart, daughter of James I of England. Four of the electors
were Catholic, three Protestant; if this could be changed, it might result in a Protestant Emperor.
When Ferdinand was elected king of Bohemia in 1617, he gained control of its electoral vote;
however, his conservative Catholicism made him unpopular with the largely Protestant nobility,
who were also concerned at the erosion of their rights. In May 1618, these factors combined to
bring about the Bohemian Revolt.[33]

Phase I: 1618 to 1635[edit]

The Bohemian Revolt[edit]
Main article: Bohemian Revolt
The Jesuit educated Ferdinand once claimed he would rather see his lands destroyed than
tolerate heresy for a single day. Appointed to rule the Duchy of Styria in 1595, within eighteen
months he eliminated Protestantism in what was previously a stronghold of the Reformation.
[34]
 Focused on retaking the Netherlands, the Spanish Habsburgs preferred to avoid antagonising
Protestants elsewhere, and recognised the dangers associated with Ferdinand's fervent
Catholicism, but accepted the lack of alternatives.[35]
Ferdinand reconfirmed Protestant religious freedoms when elected king of Bohemia in May 1617,
but his record in Styria led to the suspicion he was only awaiting a chance to overturn them.
These concerns were exacerbated when 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, the two men and their
secretary Philip Fabricius were thrown out of the castle windows, although all three survived.[36]
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 Bohemia
was one of the most prosperous areas of the Empire and its electoral vote crucial to ensuring
Ferdinand succeeded Matthias as Emperor. The combination meant their recapture was vital for
the Austrian Habsburgs but chronic financial weakness left them dependent on Maximilian and
Spain for the resources needed to achieve this.[37]
By mid-June 1619, the Bohemian army under Thurn was outside Vienna and although
Mansfeld's defeat by Imperial forces at Sablat forced him to return to Prague, Ferdinand's
position continued to worsen.[39] Gabriel Bethlen, Calvinist Prince of Transylvania,
invaded Hungary with Ottoman support, although the Habsburgs persuaded them to avoid direct
involvement; this was helped when the Ottomans became involved in the 1620 Polish war,
followed by the 1623 to 1639 conflict with Persia.[40]

On 19 August, the Bohemian Estates rescinded Ferdinand's 1617 election as king, and formally
offered the crown to Frederick on 26th; two days later, Ferdinand was elected Emperor, making
war inevitable if Frederick accepted. With the exception of Christian of Anhalt, his advisors urged
him to reject it, as did the Dutch, the Duke of Savoy, and his father-in-law James. 17th century
Europe was a highly structured and socially conservative society, and their lack of enthusiasm
was due to the implications of removing a legally elected ruler, regardless of religion.[41]