Why it started
The Thirty Years’ War began as a regional religious conflict in the Kingdom of Bohemia and expanded into a general European war that drew in almost every major Continental power before its conclusion in 1648. The underlying cause was the unresolved religious settlement of the Holy Roman Empire: the 1555 Peace of Augsburg had established the principle that each prince of the Empire could determine the official religion of his territory (cuius regio, eius religio), but had recognized only Lutheranism and Catholicism; the rapidly expanding Calvinist (Reformed) Protestantism was excluded from the settlement and had no recognized legal status within the Empire.
The immediate trigger was the Bohemian succession crisis of 1617–1618. The Habsburg emperor Matthias I, childless and elderly, had nominated his Catholic and unyielding cousin Ferdinand of Styria as his successor. The Bohemian nobility, predominantly Protestant (a mixture of Hussite, Lutheran, and Calvinist), opposed the succession on the grounds that Ferdinand would undo the religious tolerances guaranteed by Matthias’s Letter of Majesty of 1609. The Bohemian estates declared Ferdinand’s election void in May 1618 and threw two of his Catholic regents (Vilém Slavata and Jaroslav Bořita of Martinice) and their secretary out of a high window of Prague Castle on 23 May 1618. The defenestration — by the standards of subsequent Thirty Years’ War atrocities a remarkably restrained event — initiated thirty years of generalized European warfare.
(The three defenestrated officials survived, having fallen onto a pile of manure beneath the window. Catholic pamphlets attributed the survival to divine intervention; Protestant pamphlets attributed it to the manure.)
The Bohemian phase, 1618–1625
The Bohemian estates declared their independence from Habsburg rule and elected Frederick V of the Palatinate, a Calvinist German prince, as their new king in August 1619. Frederick reigned briefly as “the Winter King” before his army was destroyed at the Battle of White Mountain (8 November 1620) by an imperial-Bavarian force under the Catholic League. The Habsburgs reasserted control of Bohemia, executed 27 Protestant leaders at the Old Town Square in Prague in June 1621 (the heads of 12 were displayed on the Old Town Bridge Tower for ten years), and forcibly recatholicized the Bohemian aristocracy and population over the following decade.
The Bohemian war had spread by 1620 into the Palatinate region of western Germany and brought in Spanish Habsburg forces from the Spanish Netherlands. By 1625 the Catholic and imperial side was militarily dominant across central Europe.
The Danish and Swedish phases, 1625–1635
The Danish phase opened with the intervention of King Christian IV of Denmark, a Lutheran, who entered the war in 1625 as the head of the Protestant cause in defence of his own German territorial interests. The Danish army was destroyed at Lutter in 1626 by the imperial general Albrecht von Wallenstein. Christian withdrew from the war in 1629.
The Edict of Restitution of March 1629 — the imperial decree that ordered the return to the Catholic Church of all ecclesiastical territories secularized by Protestant princes since 1552 — radicalized the conflict. The Protestant German princes, faced with substantial territorial losses, formed a new defensive league. The Swedish King Gustavus Adolphus, an outstanding military commander and a committed Lutheran, intervened on the Protestant side in summer 1630.
The Swedish phase, 1630–1635, was the most militarily creative period of the war. Gustavus Adolphus’s army — combined-arms infantry, mobile artillery, light cavalry, integrated with the local Protestant German forces — defeated the imperial-Catholic League army decisively at Breitenfeld (September 1631) and at Lützen (November 1632). Gustavus Adolphus himself was killed at Lützen, in a cavalry mêlée in the fog. He was 37. His chancellor, Axel Oxenstierna, continued the war in Sweden’s interest.
The Catholic-imperial side was militarily on the defensive through 1632–1634 but recovered with the imperial-Catholic League victory at Nördlingen (September 1634). The Peace of Prague (May 1635) ended the formal Swedish-imperial war and produced a partial religious settlement that was acceptable to the moderate German Protestants.
The French phase, 1635–1648
The Peace of Prague brought peace neither to Germany nor to the larger European conflict. France, under the de facto leadership of Cardinal Richelieu (Louis XIII’s chief minister, despite his Catholic identity prioritizing French strategic interests over Catholic religious solidarity), declared war on Habsburg Spain in May 1635. France allied with Sweden, the Dutch Republic, and the smaller Protestant German powers against the combined Spanish and Austrian Habsburg states. The war continued for thirteen further years.
The French phase was the longest, the most demographically catastrophic for Germany, and the least religiously motivated. The Catholic French and Catholic Habsburgs fought each other for state-strategic reasons. The German territories were the primary battleground; the armies (mercenary on both sides, increasingly dependent on local plunder and unpaid for years at a time) consumed the agricultural and human resources of central Europe systematically. The war’s worst single atrocity — the imperial sack of Magdeburg in May 1631, in which approximately 20,000 of the city’s 25,000 inhabitants were killed in a three-day systematic massacre — gave the German language the verb magdeburgisieren, “to magdeburgize.”
The Swedish and French armies fought a series of battles in Bavaria, Bohemia, the Rhineland, and Westphalia from 1635 onward. The military situation by 1645 was a Protestant-French stalemate that neither side could decisively resolve. Both sides were exhausted financially and demographically. Peace negotiations began at Münster and Osnabrück (two cities in Westphalia, chosen because they could be reached independently by both Catholic and Protestant negotiators) in 1644.
The Peace of Westphalia
The Peace of Westphalia was the negotiated settlement, signed on 24 October 1648 as two separate but linked treaties (the Treaty of Münster, between the Holy Roman Empire and France, and the Treaty of Osnabrück, between the Holy Roman Empire and Sweden, with simultaneous Dutch-Spanish settlement at Münster). It was the largest single international diplomatic conference of the early-modern period; it involved 109 delegations and produced approximately 130 treaty documents.
The substantive provisions were: territorial confirmations of the existing balance of religious power, with the principle of cuius regio, eius religio extended to include Calvinism (not just Lutheranism and Catholicism); the formal independence of the Dutch Republic from Spain (after a century of effective de facto independence); the formal independence of the Swiss Confederation from the Holy Roman Empire; substantial French territorial gains in Alsace; substantial Swedish territorial gains in northern Germany; the formal recognition of the autonomy of the German princes from the Habsburg emperor, effectively converting the Holy Roman Empire from a centralized monarchy into a confederation of approximately 350 quasi-sovereign territorial states; and an amnesty for all wartime political actions.
The Peace of Westphalia is the foundational treaty of the modern European state system. The principle of sovereign territorial states, the principle of non-intervention in the internal religious affairs of other states, and the institution of permanent diplomatic representation between states all descend from the 1648 settlement. The standard scholarly term for the modern international order — the Westphalian system — is taken from this peace.
What it cost
The Thirty Years’ War was the demographic catastrophe of early-modern Europe. Conservative modern estimates of the German population in 1618 are approximately 21 million; in 1648, approximately 13 million — a 38 percent reduction. The mortality was unevenly distributed: some territories (Mecklenburg, parts of Württemberg, Bohemia) lost 60 percent or more of their population; some territories (parts of the Rhineland and the Netherlands) escaped with only modest losses. Most of the mortality was from disease, famine, and economic collapse rather than direct military action.
The war’s intellectual and cultural consequences in central Europe were severe. The German academic tradition that had produced the Scientific Revolution at Wittenberg and Heidelberg in the early 17th century was severely disrupted. The witch-hunt panic of the early 17th century — including the trial of Johannes Kepler’s mother for witchcraft in 1620 — was substantially enabled by the war’s atmosphere of religious paranoia and social dislocation.
The war’s end produced the substantive end of state-organized religious warfare in Europe. There would be no further large-scale Catholic-Protestant European wars after 1648. The English Civil War (1642–1651), the Anglo-Dutch Wars (1652–1674), the War of the Spanish Succession (1701–1714), and the European wars of the 18th century would all be fought for state-political reasons unrelated to confessional religious dispute. The European Wars of Religion that had begun with the Reformation of 1517 ended in 1648.