The Thirty Years' War: Map of Europe, 1618–1648

The Holy Roman Empire was a patchwork of more than a thousand territories, from kingdoms and electorates to free cities, under an elected emperor who in practice was always a Habsburg. The Reformation had split it between Catholic and Protestant princes. The Peace of Augsburg of 1555 let each prince choose his state's religion, but it recognised only Lutherans, not the growing number of Calvinists, and by 1608–1609 the princes had formed rival armed leagues: the Protestant Union and the Catholic League.

The spark came in Bohemia. Its mostly Protestant nobles feared that their new king, the zealous Catholic Ferdinand of Styria, would take away their religious freedoms. On 23 May 1618 a group of them threw two royal governors and a secretary out of a window of Prague Castle. All three survived the fall, but the Defenestration of Prague began a rebellion.

The rebels deposed Ferdinand, who became Holy Roman Emperor as Ferdinand II in 1619, and offered the crown of Bohemia to Frederick V, the Calvinist Elector Palatine. It was a fatal choice. On 8 November 1620 an Imperial and Catholic League army routed Frederick's forces at the White Mountain outside Prague. Frederick fled, mocked as the "Winter King" for his single season on the throne. Bohemia was forcibly re-Catholicised, Protestant nobles were executed or exiled and their lands handed to loyalists. Spanish and Bavarian troops then overran Frederick's own Rhineland Palatinate, and in 1623 his electoral vote went to Maximilian of Bavaria.

Alarmed by Catholic victories, King Christian IV of Denmark, who was also a German duke, entered the war in 1625. He was beaten at Lutter in 1626 by Tilly, the Catholic League's general. The emperor had meanwhile found a new commander, the Bohemian nobleman Albrecht von Wallenstein, who raised a huge private army that lived off the lands it occupied. By 1628 Wallenstein held most of northern Germany and Jutland and was only stopped at the Baltic port of Stralsund. At the height of his power, in March 1629, Ferdinand issued the Edict of Restitution, ordering Protestants to return all church lands taken since 1552. It united the Protestants against him.

In the summer of 1630 Gustavus Adolphus, king of Sweden, landed in Pomerania with about 18,000 men, paid partly by Catholic France, which wanted the Habsburgs weakened. Before he could get there, Tilly's army stormed the Protestant city of Magdeburg on 20 May 1631. Fire and massacre killed about 20,000 of its 25,000 inhabitants, the worst atrocity of the war, and the outrage drove Saxony and Brandenburg into Sweden's arms.

On 17 September 1631 Gustavus smashed Tilly at Breitenfeld near Leipzig, and in the next year he swept through Franconia and the Rhineland into Bavaria, as the blue line shows. The recalled Wallenstein met him at Lützen on 16 November 1632; the Swedes won the battle, but Gustavus was killed in the fog of the cavalry fighting. Wallenstein, suspected of treason, was murdered by his own officers at Eger in February 1634. That September a Spanish-Imperial army crushed the Swedes at Nördlingen, and in 1635 most German Protestant princes made their peace with the emperor at Prague.

The peace among the Germans did not end the war, because France's chief minister, Cardinal Richelieu, would not let the Habsburgs win. In May 1635 Catholic France declared war on Catholic Spain, and the war became a struggle between the great powers fought on German soil, and also in the Low Countries, Italy, Catalonia and Portugal. The French victory over the Spanish army at Rocroi in 1643 broke the reputation of the Spanish infantry, and Swedish armies ranged across Germany and Bohemia, winning at Wittstock in 1636, at Breitenfeld again in 1642 and at Jankau in 1645.

Armies of tens of thousands lived by plunder. They spread plague and typhus, burned villages and drove peasants from their fields, so most of the dead died of hunger and disease rather than in battle. Some parts of Germany lost more than half their people.

Peace talks opened in 1643–1644 in two Westphalian towns: the Catholic powers met in Münster and the Protestants in Osnabrück. On 30 January 1648 Spain signed the Peace of Münster with the Dutch, recognising the independence of the Dutch Republic after 80 years of war. The emperor signed the treaties of Osnabrück and Münster on 24 October 1648, while Swedish troops were besieging Prague, ending the war in the Empire.

Under the peace, Sweden gained Western Pomerania, Wismar and the bishoprics of Bremen and Verden, plus five million thalers; France kept Metz, Toul and Verdun and gained the Habsburg lands and rights in Alsace; Brandenburg gained Eastern Pomerania and Magdeburg; Bavaria kept the Upper Palatinate and its electoral vote, while a new eighth electorate was created for the Palatinate. Calvinism was recognised alongside Lutheranism and Catholicism, and the German princes gained the right to make their own alliances. Switzerland's independence was also formally recognised. Compare 1618 and 1648 on the map to see the changes.

Because it treated the many states as sovereign parties, the Peace of Westphalia is often called the birth of the modern system of sovereign states, "Westphalian sovereignty". Historians now debate how much the treaties themselves meant this, but the label has stuck.