Swedish intervention in the Thirty Years' War began in July 1630 when troops under Gustavus Adolphus landed in Pomerania. Under his leadership, the Protestant cause, previously on the verge of defeat, won several major victories and changed the direction of the war.
Following the Edict of Restitution by Emperor Ferdinand II on the height of his and the Catholic League's military success in 1629, Protestantism in the Holy Roman Empire was seriously threatened. In July 1630, King Gustav II Adolf of Sweden landed in the Duchy of Pomerania to intervene in favor of the German Protestants. The Swedish armies achieved several victories against their Catholic enemies, including the victory at the Battle of Breitenfeld in 1631. Gustav II Adolf was killed in battle at Lützen, southwest of Leipzig, and two years later the decisive defeat at Nördlingen in 1634 threatened continuing Swedish participation in the war. In consequence, the Emperor made peace with most of his German opponents in the Peace of Prague – essentially revoking the Edict of Restitution – while France directly intervened against him to prevent the Habsburg dynasty from gaining too much power at its eastern border.
Sweden was able to fight on until the Peace of Westphalia in 1648 in which the Emperor was forced to accept the "German liberties" of the Imperial Estates and Sweden obtained Western Pomerania as an Imperial Estate.
Religious and political background: The Bohemian Revolt
The Thirty Years' War was a religious conflict between Protestants and Catholics in Germany. It originated in the co-mingling of politics and religion that was common in Europe at the time. Its distal causes reside in the previous century, at the political-religious settlement of the Holy Roman Empire known as the Peace of Augsburg. The peace was an agreement between Charles V, Holy Roman Emperor and the Protestant powers of the Holy Roman Empire in the 16th century. It established the legitimacy of Lutheranism in Germany and allowed Dukes and high lords to determine the faith of their fiefdom as well as to expel non-conforming subjects from their territory, the principle known as Cuius regio, eius religio. Additionally, it permitted subjects of a different religion to peaceably move to land where their practices would be recognized and respected. There were also clauses in relation to ecclesiastical lords. When prelates who ruled an ecclesiastical fiefdom converted, the expectation was placed upon them to resign their temporal privileges. Some of these treaty stipulations would be violated on various occasions, as was the case with Gebhard Truchsess von Waldburg, Archbishop-Elector of Cologne. Despite various attempts to violate the provisions in the Peace of Augsburg no general European or German conflagration would erupt as a result of the violations. At the end of the conflicts, it was agreed that the provisions of the Peace of Augsburg would once again be adhered to. "All that the Lutheran church gained by the Peace of Augsburg was toleration; all that the [Roman] church conceded was a sacrifice to necessity, not an offering to justice" says one historian. However, the Peace of Augsburg could never be anything but a temporary cessation of hostilities. Its provisions contained an addendum that declared that it would only become active without reservation upon the meeting of a general council and a final attempt at the reunion of the two confessions. There was no reason to believe these would ever happen unless the Lutherans were forced to do so.
Although genuine ideological differences did drive German Princes to convert, the primary motivation of many was often the acquisition of easy riches and territory at the expense of their defenceless Catholic neighbours and subjects. Princes would convert on the grounds that they would be empowered to seize precious land and property from the Roman Catholic Church, and turn that wealth to their own enrichment.
The Protestants understood and accepted as an article of faith that they would have to unite against the Roman Catholic Church to preserve themselves against Catholic encroachment and eventual Catholic hegemony. However, the Protestants were divided. Lutherans held to articles of faith that were mutually exclusive with the articles espoused by Calvinists. The Roman Catholic Church did everything in its power to sow controversy and intrigue between the two major Protestant factions. As a result, there was no political unity of German Protestant states that could coordinate actions against a Catholic interloper.
It was regularly maintained by both religious parties that the other regularly encroached on the spirit or the letter of the Peace of Augsburg. Indeed, it was understood by Protestants that Catholic officials (especially imperial or church officials) were vicious and jealous of the privileges acquired by the Protestants and would do anything in their power to harm the Protestant cause. It was established by practice that the Pope had the power to relieve members of his flock of the most solemn oaths, and it was a matter of principle among Catholics that faith was never to be kept with heretics. On the other hand, Catholics maintained a similar understanding of the Protestants. The avarice displayed by Protestants for Church property could not fail to go unnoticed by even the most indulgent Catholic observer. With such mutual antipathy prevailing between the Protestants and Catholics of Germany, nothing that could would fail to be misunderstood.
The Thirty Years' War arose out of a regional dispute between Bohemian Protestants and their Habsburg monarchs. Rudolf II, Holy Roman Emperor was an obstinate and stubborn monarch. His policies forced him into an increasingly weak position with his heterogenous subjects, his court and his family. Forced to make concessions to his Hungarian subjects in order to appease them for his indecisive war with the Ottoman Empire, Rudolf ceded his Hungarian, Austrian and Moravian holdings to his brother, Matthias. Seeing weakness and discord in the ranks of their German overlords, his Bohemian subjects revolted. In 1609, Rudolf granted them concessions with the Letter of Majesty which included religious tolerance and a church for the Bohemian Estate controlled by the Protestant nobility. When the Protestant estates in Bohemia requested even more liberties, Rudolf sent in an army to quiet them. However, Matthias seized his brother at the request of the Protestant Bohemians, only releasing him once he abdicated his Bohemian crown to Matthias. Rudolf II died a couple of months later in 1612, at which point his brother Matthias acquired the rest of his titles, including that of Holy Roman Emperor.
Being without heirs, in 1617 Matthias had his cousin Ferdinand of Styria elected King of Bohemia, a gesture which amounted to naming him as his successor. They were related through their paternal grandfather Ferdinand I. Ferdinand of Styria, or Ferdinand II as he was to become known, was an ardent follower of Catholicism and the Counter-Reformation and not likely to be as willing to compromise as his two cousins and predecessors on the Bohemian throne were or had been forced to do by circumstance. Ferdinand had not received his Bohemian throne in a weak position, as Mathias or Rudolf had. Matthias had acceded to the Protestants' demands to allow Protestant religious facilities to be constructed on Bohemian crown lands. Ferdinand was to reverse the construction of many of these facilities on his ascension to the Bohemian crown, and when the Bohemian estates protested, he dissolved the Bohemian assembly.
The Third Defenestration of Prague was the immediate trigger for the Thirty Years' War. In May 1618, the three estates of the dissolved Bohemian assembly gathered in Prague, the capital of the Bohemian kingdom. Protestant noblemen led by count Jindřich Matyáš Thurn, recently stripped of his title as castellan of Karlstadt by the Emperor, stormed Prague Castle and seized two imperial governors appointed by Ferdinand, Vilem Slavata of Chlum and Jaroslav Borzita of Martinice, and two imperial secretaries. The noblemen held an illegal trial immediately, found the Imperial officials guilty of violating the Letter of Majesty, and threw them out of a third-floor window of the Bohemian Chancellery, though they survived.
The implications of the event were immediately apparent to both sides, who started to seek support from their allies. The Bohemians were friendless, against a powerful monarch of Europe who had many and powerful allies, and who was a scion of one of the most powerful dynasties in Europe who stood to inherit the entirety of the Emperor's dominions. The Bohemians made offers to the Duke of Savoy, Elector of Saxony (the preferred candidate) and even to the Prince of Transylvania. They also sought admission into the Protestant Union, a coalition of German Protestant states formed to give a politico-military unity to the divided German Protestants. The Elector of Saxony's refusal of the Bohemian crown made the Elector Palatine the most senior Protestant available to the Bohemians. In addition to being a Protestant, albeit a Calvinist, Frederick V was married to Elizabeth Stuart and was thereby a son-in-law of the King of England, indisputably the most powerful Protestant monarch, and whose aid it was not unreasonable to hope for.
However the act of unseating Ferdinand – the legitimately chosen monarch of Bohemia- put the Bohemian revolt in a difficult position with the other political powers of Germany and Europe. John George I of Saxony refused the election, and discouraged the nascent revolt. In September of the same year, the Protestant Union met and called on Frederick not to intervene in the conflict. The Dutch Republic, Charles Emmanuel of Savoy and even the Republic of Venice – a traditional enemy of the Pope – sent letters to Frederick informing him that they would not offer him assistance if he accepted the Bohemian crown – but nonetheless, he did.
Background in Sweden
Gustavus Adolphus had been well informed of the war for some time, but his hands were tied because of the constant enmity of Poland. The Polish royal family, the House of Vasa asserted its right of primogeniture to the Swedish throne – which indeed it had once held. However, when Sigismund III Vasa was elected by the nobles of the Polish–Lithuanian Commonwealth, he was elected on the condition that he be a Roman Catholic. Which he was, as he had a mother who was Roman Catholic and had abandoned the religion of his predecessors, however Lutheranism was the primary religion of Sweden, and had by then established a firm grip on the country. It was not solely the result of religious sentiment that Sweden converted. Notably, one of the reasons that Sweden had so readily embraced it was because converting to Lutheranism allowed the crown to seize all the lands in Sweden that was possessed by the Roman Catholic Church. As a result of this seizure and the money that the crown gained, the crown was greatly empowered. In spite of this, he retained his mother's Roman Catholicism as his religion. Although he guaranteed the rights of this religion to the people in his Swedish domains, this was a subject of great contention for the kingdom. Sigismund's right to the throne became a further subject of contention due to his support for the Counter-Reformation. After Sigismund's defeat in the Battle of Stångebro, the Swedish nobility demanded that he rule Sweden from Sweden. Despite their demands, Sigismund returned to his Polish capital, Warsaw, and was deposed from the Swedish throne in 1599.
Gustavus Adolphus' father, Charles IX of Sweden – the uncle of Sigisimund – also a Vasa, was awarded the throne, in part because he was an ardent Lutheran. Soon after, Sweden became engaged in wars with the Kingdom of Denmark–Norway and the Tsardom of Russia. Also, Sigismund III never renounced his claim to the Swedish throne, and for many years the primary direction of Poland's foreign policy was directed at reacquiring it. As a result, Sweden was hard pressed on almost all of its borders.
Charles IX died in 1611, without achieving any conclusive result in Sweden's wars during the six years of his reign. At the age of just 17, Gustavus was granted a special dispensation to assume the Swedish crown – and thereby inherited his father's conflicts.
The surrounding powers smelled blood, assuming that such a youth could not maintain the gains that the father had won for Sweden. However, Gustavus had first entered the army at the age of 11, and had first-hand knowledge how to govern a kingdom. His training at statecraft had begun at the same age, when later that year his father permitted him to sit in on meetings of the council of state. The neighbouring powers had not assessed the new king accurately.
The new king was able to bring about conclusive results to the conflicts he had inherited. By 1613 Gustavus had compelled the Danes out of the war after landing on Swedish territory only 10 kilometres (6 mi) from the capital. By 1617 he had forced Russia out of the wars and compelled her to cede territory to Sweden.