The war began with the resistance and eventual revolt of Protestant nobles in Bohemia, which was under Hapsburg domination, against the Catholic king Ferdinand (later Holy Roman Emperor Ferdinand II). It spread through Europe because of the constitutional frailty of the Holy Roman Empire, the inability of the German states to act in concert, and the ambitions of other European powers.

The revolt began in Prague, where two royal officers were hurled from a window by Protestant members of the Bohemian diet—the so-called Defenestration of Prague (May, 1618). Ferdinand was declared deposed and the Bohemian throne was offered to Frederick V, the elector palatine. Revolt also appeared in other Hapsburg dominions, especially under Gabriel Bethlen in Transylvania. Duke Maximilian I of Bavaria, with the army of the Catholic League under Tilly, helped the imperial forces defeat the Bohemians at the White Mt. near Prague (Nov., 1620). John George of Saxony, a leading German Protestant prince, supported Ferdinand. Frederick, ever afterward called the Winter King, had lost his brief hold on Bohemia. The war continued in the Palatinate, and severe repression began in Bohemia.

Mansfeld and Christian of Brunswick led the revolutionary forces in the Palatinate. Frederick expected aid from his father-in-law, James I of England, but got no effectual help. The Palatinate was taken by Tilly; he won at Wimpfen and Höchst (1622). Frederick's lands were confiscated by the emperor, and the Upper Palatinate and the electorate were conferred on Maximilian of Bavaria. The imperialist victory at Stadtlohn (1623) practically ended one phase of the war.

The new phase saw the German war expanded into an international conflict. Christian IV of Denmark came into the fighting, principally because of his fear of the rise of Hapsburg power in N Germany; he openly avowed religious motives but hoped also to enlarge his German possessions. England and the United Provinces gave a subsidy to aid the opponents of the Hapsburgs, and England sent a few thousand soldiers. Christian IV advanced into Germany. The emperor's cause was advanced by the work of Wallenstein, who gathered an effective army and defeated Mansfeld at Dessau (1626). A little later the Danish king was soundly defeated by Tilly at Lutter.

The imperial armies swept through most of Germany. Wallenstein went into Jutland and vanquished the Danes but failed before Stralsund (1628). In 1629, Denmark, by the Treaty of Lübeck, withdrew from the war and surrendered the N German bishoprics. The Edict of Restitution (1629), issued by Ferdinand II, attempted to enforce the ecclesiastical reservation of the Peace of Augsburg and declared void Protestant titles to lands secularized after 1552; its full application would have had a disastrous effect on German Protestantism and naturally aroused the Protestant states to determined, if at first latent, hostility.

Gustavus II (Gustavus Adolphus) of Sweden now came into the war. His territorial ambitions had embroiled him in wars with Poland, and he feared that Ferdinand's maritime designs might threaten Sweden's mastery of the Baltic. Moved also by his Protestantism, he declared against the emperor and was supported by an understanding with Catholic France, then under the leadership of Cardinal Richelieu. Swedish troops marched into Germany. Meanwhile, Ferdinand had been prevailed upon (1630) to dismiss Wallenstein, who had powerful enemies in the empire. Tilly now headed the imperial forces. He was able to take the city of Magdeburg while the Protestant princes hesitated to join the Swedes. Only John George of Saxony, vacillating in his support between Tilly and the Swedish king, joined Gustavus Adolphus, who offered him better terms.

The combined forces crushed Tilly at Breitenfeld (1631), thus winning N Germany. Gustavus Adolphus triumphantly advanced and Tilly was defeated and fatally wounded in the battle of the Lech (1632). Wallenstein, recalled with some pleading by the emperor, took the field. He defeated the Saxon forces and later met the Swedish forces at Lützen (Nov., 1632); there the imperialists were defeated, but Gustavus Adolphus was killed and the anti-Hapsburg troops were disorganized. Wallenstein after his great defeat remained inactive and entered into long negotiations with the enemy. Meanwhile, the able anti-imperialist general, Bernhard of Saxe-Weimar, stormed Regensburg (1633).

Wallenstein was murdered in 1634 by imperialist conspirators. Soon afterward the imperial forces under Gallas defeated Bernhard at Nördlingen (Feb., 1634). Germany was in economic ruin, her fields devastated and blood-soaked. There was strong feeling in Germany against the foreign soldiers that overran the land. A general desire for peace led to the Peace of Prague (1635). This agreement drastically modified the Edict of Restitution, thus helping to reconcile Catholics and Protestants. It was accepted by almost all the German princes and free cities. A united imperial army was to move against the Swedish troops in Germany. A general peace seemed to be forthcoming, but Richelieu was unwilling to see the Hapsburgs retain power.

France entered openly into the war in 1635. Oxenstierna, the Swedish chancellor, anxious to preserve Sweden's hold in Germany, supported Richelieu. The final stage of the Thirty Years War began. The war now occupied most of Europe, with fighting in the Low Countries, where the United Provinces and France opposed Spain; in Italy, where France and Spain struggled for power; in France; in Germany; in the Iberian peninsula, where Portugal revolted against, and France attacked, Spain; and in the North, where Denmark opposed Sweden.

The Austrian forces went into France and achieved some success, but this was temporary. For the most part this period of the war was disastrous for the empire. Bernhard of Saxe-Weimar and the Swedish general, Baner, were victorious in Germany. In 1636 Baner won a notable victory at Wittstock. Bernhard conducted a series of brilliant campaigns, culminating in the capture of Breisach (1638). Bernhard died in 1639, Baner in 1641. Meanwhile, Emperor Ferdinand II was succeeded by Ferdinand III (1637). In 1642 Richelieu died; his successor, Cardinal Mazarin, continued the established French policy. Germany was exhausted.

Peace negotiations were begun before 1640, but the intricate diplomacy proceeded slowly and haltingly. Meanwhile, the empire was reduced by the armies of the Swedish Torstensson, Louis II de Condé, and Turenne. Torstensson defeated the imperialists at Breitenfeld (1642), defeated Gallas after going north to subdue Danish opposition, then won a climactic victory over Hatzfeldt at Jankau (1645). Meanwhile, Condé had destroyed the flower of the Spanish infantry at Rocroi (1643); in 1645 he and Turenne (after a severe defeat) were victorious near Nördlingen. Austria had been stripped of all conquests and her enemies were at the very door of Vienna. Austria's strongest ally, Bavaria, was overrun. The Swede Wrangel and the Frenchman Turenne were carrying on a successful campaign when the long-delayed peace was obtained (see Westphalia, Peace of).

The general results of the war may be said to have been a tremendous decrease in German population; devastation of German agriculture; ruin of German commerce and industry; the breakup of the Holy Roman Empire, which was a mere shell in the succeeding centuries; and the decline of Hapsburg greatness. The war ended the era of conflicts inspired by religious passion, and the Peace of Westphalia was an important step toward religious toleration. The incredible sufferings of the German peasantry were remembered for centuries. The political settlements of the peace were to the disadvantage of Germany as well as the Hapsburgs. The estrangement of N Germany from Austria, then begun, was to continue for more than two centuries.

See studies by S. R. Gardiner (1874, repr. 1968); C. R. L. Fletcher (1903, repr. 1963); C. V. Wedgwood (1962, repr. 1981); S. H. Steinberg (1966); G. Pages (tr. 1970); J. V. Polisensky (tr. 1971); and G. Parker (1988). Many of the songs and writings of the Thirty Years War have been collected.