- A geopolitical war between the House of Austria (the Habsburg Holy Roman Emperors Ferdinand II and Ferdinand III together with their Spanish cousin Philip IV) and their opponents, Denmark, the United Provinces, France and Sweden.
- A German civil war. The principalities that made up Germany took up arms for or against the Habsburgs or, most commonly, both at different times during the war’s 30 years.
- A religious war among Catholics, Lutherans and Calvinists. Ferdinand II and, to a lesser degree, his primary ally Maximillian I represented the re-Catholicizing zeal of the Jesuit Counter-reformation, while Frederick V of the Palatinate represented the equally militant forces of Calvinism.
- A war that was difficult to control as power increasingly rested with the army commanders: Tilly, Spinola, Wallenstein.
In the last decades of the sixteenth century attitudes hardened on both sides. During the reign of the tolerant Maximilian II, Calvinism and Anabaptism spread freely in the eastern reaches of the Habsburg lands (lands outside the Peace of Augsburg), notably in Austria, Bohemia, and Hungary. In 1609 Rudolf II’s Letter of Majesty granted religious autonomy to the Estates of Bohemia. Over subsequent years, however, the Habsburgs gradually began to chip away at these concessions. The Catholics commanded a majority in most of the organs of government; the Protestants came to distrust these bodies and the machinery of government began to break down. Catholics and Protestants formed armed alliances to preserve their rights: the Catholic League under Maximilian I of Bavaria and the Protestant Union under the Elector Frederick V, the Calvinist ruler of the Palatinate, the son-in-law of James I. A further complicating factor was that the truce between Spain and the Dutch (‘the Twelve Years’ Truce') was due to expire in 1621, and the Dutch war for their liberation from the Spaniards was expected to start up. Spain would consequently have a major interest in Germany, especially in the areas bordering the Spanish Netherlands. The Bohemian Phase 1618-21 Bohemia occupied an anomalous position. The King of Bohemia was also the head of the Habsburg family and Prague was an alternative capital to Vienna. The King usually became Holy Roman Emperor. But the King inherited his throne, whereas the Emperor was elected by the seven Electors of whom the King of Bohemia was one.
In 1617 the Jesuit-educated Archduke Ferdinand of Styria (left) was recognized as King of Bohemia, and King of Hungary the following year. He clearly intended to destroy the religious compromise of 1609 in Bohemia. His opponents called a Protestant assembly in Prague in 1618. The Emperor Matthias rejected their petition and in response the two governors of Prague and their secretary were thrown out of a high window of the Hradschin Palace in the Defenestration of Prague on 23 May 1618.
After the death of the Emperor Matthias in March 1619 the Estates General of Bohemia formally
deposed Ferdinand and chose Frederick V of the Palatinate as their King. (He is shown right in his Bohemian coronation robes.) This was a direct challenge to the Emperor and to Spanish imperial interests on the Rhine. It opened up a dispute not merely between Bohemia and the Habsburgs but within the Protestant areas of the Empire.
On 28 August 1619 Ferdinand was unanimously elected Emperor Ferdinand II. He was supported by his Spanish relatives, the Lutheran Elector of Saxony, and by Maximilian of Bavaria. A Catholic army of 20-30,000 commanded by the Flemish born Johann Tilly, marched into Bohemia in September 1620. In November, after fighting that lasted for only an hour, the Bohemians were utterly defeated near Prague at the battle of the White Mountain. Frederick and Elizabeth fled, and Habsburg role was re-imposed with great harshness in Prague.
Over the next decades, just over half the landed estates in Bohemia were expropriated. In 1627 Bohemia lost its independent religious existence and Protestant preachers were expelled from Lower Austria. An estimated 150,000 people emigrated from Bohemia. This took place against a background of economic chaos, rampant inflation and taxation riots.
The Palatinate Phase 1621-24
Simultaneously the war extended westwards when the Spaniards under the Italian general Ambrogio Spinola invaded the Palatinate. They did so in order to protect the communication routes between the Spanish bases in northern Italy, particularly Milan, and the Spanish Netherlands, known as the ‘Spanish road’. By 1622 it had been conquered and the electoral title was transferred to Maximilian of Bavaria. This enabled the Spaniards to secure the land route from their territories in Northern Italy to their lands in the Spanish Netherlands, a development that alarmed the government of Louis XIII.
Wallenstein
In 1625, with the Palatinate war over, the emperor accepted the offer from his most brilliant general, the Bohemian Albrecht von Wallenstein of a mercenary army that would be under Wallenstein’s command. This introduced and new and unpredictable force into the conflict. 
Wallenstein had been born a Lutheran but he changed his religion. He built up an immense fortune through marrying a wealthy and dying widow and becoming a shareholder in a company that produced a new Bohemian currency. He bought up confiscated Protestant lands and was soon in a position to make loans to the Emperor. He entered his military service by contracting to supply 20,000 armed men. His own organization supplied food and equipment, but the pay was erratic, and when they were not paid the soldiers lived off the countryside.
The Danish Phase 1625-29
In 1625 the United Provinces, found a champion in the Lutheran ruler, Christian IV of Denmark, who also had extensive possessions in northern Germany. He put himself at the head of a mercenary army hired for the defence of Protestantism, but in 1626 he was defeated by Tilly at Lutter in Lower Saxony, after which Wallenstein’s army occupied Jutland. In 1629 Wallenstein who had been granted plenipotentiary powers by the Emperor, persuaded Christian to accept a generous peace at Lübeck in 1629. This restored the status quo in return for the king’s promise not to intervene again.
This was a dark moment for the Protestant cause. The Habsburg position was as strong as it would ever be. With plans for control of the southern coast of the Baltic in alliance with Poland, it seemed likely that they might block Dutch trade in the Baltic. For a while it looked as if Protestantism might collapse completely in northern Germany.
Emboldened by victory, the Emperor issued the Edict of Restitution on 6 March 1629. This required the return of all church property expropriated since 1552 to the heirs of the former owners, and led to forcible seizures of territory; Magdeburg, a Protestant city, was returned to Catholic control. This was a grave mistake on Ferdinand's part, as he had alienated his potential Protestant allies. Germany was thrown into chaos. At the same time Spain was distracted by other commitments. In particular the war for the succession to the duchy of Mantua was a powerful distraction. Wallenstein’s enormous accretion of power was also causing resentment. By 1630 he was in command of c. 130,000 troops and conducting his own secret negotiations. On 13 August 1630, on Maximilian’s urgings, Ferdinand reluctantly dismissed him, just, as the Protestant cause was beginning to recover.
The Swedish Phase 1629-34
Concerned by growing Habsburg power along the Baltic, the 35-year-old Gustavus Adolphus of Sweden invaded northern Germany with a largely mercenary army in July 1630. Gustavus had been king since he was sixteen, and had been involved in wars with Denmark, Russia, and Poland. He was remarkable as a military leader and an administrator, and he had managed to establish his authority over his unruly nobles. He had built up the most professional army of the period. However, Gustavus was not welcomed by his fellow Lutherans, who had well-founded suspicions of his ultimate intentions: his sole significant allies were the French, who subsidized his army from 1631
The destruction of Magdeburg: Magdeburg in Lower Saxony had been in revolt since the Edict of Restitution. Tilly besieged the city but Gustavus Adolphus made the mistaken decision not to try to relieve it. Before the city could surrender, the city was stormed and taken on 20 May 1631. Only 5,000 of the30,000 population survived the subsequent sacking. By 22 May the cathedral was the only building of note left. A Te Deum was sung there and the city was renamed Marienburg. This was a profound shock to the Protestant cause.
The battle of Breitenfeld: When the Imperial forces moved against Saxony, the Elector of Saxony threw in his lot with the Swedes On 6 September Gustavus (above) defeated Tilly at Breitenfeld, near Leipzig in the largest battle of the war (Gustavus had 42,000 men, Tilly 35,000). This battle destroyed imperial ambitions in the Baltic. Tilly was forced south and much of central Germany lay exposed to Swedish occupation. He was killed at the River Lech when he failed to stop the invasion of Bavaria. The Emperor had no choice but to recall Wallenstein.
The battle of Lützen: With the Swedes now occupying half the Empire, they became very unpopular and there was deep suspicion of Sweden’s intentions. The German princes feared for their independence and this made them unreliable allies for Gustavus. In 1632 Gustavus marched against Bavaria and laid it waste. Wallenstein marched his towards Leipzig where he planned to spend the winter. Gustavus raced against him and on 16 November launched an immediate assault at Lützen, himself leading the cavalry charge. The battle was inconclusive, but Gustavus was killed. His heir was his six-year-old daughter, Christina. Swedish foreign policy was in the hands of the Chancellor, Axel Oxenstierna
The death of Wallenstein: In spite of the defeat, Wallenstein’s prestige was enormous. He was now a politician in his own right, negotiating with France, Sweden, and the Protestant electors. On 23 February 1634 he was charged with treason. Two days later he was assassinated by English and Scottish officers in the imperial army.
The battle of Nördlinden: The Imperial and Spanish armies joined and inflicted a crushing defeat on the Swedes at Nördlingen in Bavaria in September 1634. The Swedes, who were also involved in a war against Poland, were outnumbered 2:1. All the Swedish gains in southern Germany were lost. There were to be no more decisive battles in the war.
The Peace of Prague: However, the Swedish intervention had saved German Protestantism, and the Empire could never be the same again. On 30 May 1635 the compromise Peace of Prague between the Emperor and Johann Georg, the Elector of Saxony, suspended the Edict of Restitution, though it would take years of fighting before the Emperor finally recognised political and religious realities. But most of the German Protestant princes were now reconciled to the Emperor and the war was increasingly fought between foreign powers on the soil of Germany.
The French Phase 1634-48
In May 1635 France declared war on Spain and increased the scope of French interventions in the Empire. France was in alliance with Sweden and the German Protestants. Richelieu’s aim was to secure Alsace and the frontier towns. France pursued the risky strategy of fighting on two fronts, but Spain was in decline.
France took control of Alsace and much of the Rhineland. Richelieu died in 1642 but his policies were continued by Cardinal Mazarin. On 19 May 1643 the Spaniards were defeated at Rocroi in Flanders. However this victory did not end the fighting.
Gradually the Imperial forces were weakened. The Peace of Prague began to break down. In 1640 the new Elector of Brandenburg, Frederick William (‘the Great Elector’) withdrew from the settlement, while the Swedes took over or neutralized northern Germany and carried the war into Bohemia. The revived Swedish army captured Leipzig in 1642. In 1647 Bavaria was devastated by the French and Swedish armies.
The Peace of Westphalia
Over the last four years of the war, the parties were actively negotiating in two small towns in Westphalia: Osnabrück (for the Protestant parties) and Münster (for the Catholic parties). On 24 October 1648 the Peace of Westphalia (the collective name for the two treaties) was signed, ending the Thirty Years War.
- The treaty confirmed that the Empire would be a confederation, with each prince given territorial sovereignty over foreign policy, though this was not be used against the Emperor. Each prince would retain power to control both public and private church life in his own territory.
- Calvinists were given recognition alongside Lutherans, but other Protestants were not.
- However, in the Habsburg territories of Bohemia and Austria the Emperor was given a nearly free hand to re-impose Catholicism.
- Charles Louis, the son of Frederick, was restored to the Palatinate. The Swedes received a large cash indemnity and control over western Pomerania, Bremen and Verden in Lower Saxony.
- The French received rights over Alsace and the upper Rhine, though the exact nature of these rights remained unclear. The Spaniards had to abandon hope of land bridges to the Netherlands and the Austrian Habsburg territories.
- It caused huge disruption and misery, especially in the German lands.
- It bankrupted many of the participating states. Faced with the threat of unrest and revolt, governments strengthened the military.
- It killed off the idea of the forcible religious reunification of Europe: the rulers had to accept reluctantly that confessional boundaries were there to stay.
