Thirty Years' War
Structural origins

The 1552 Peace of Passau ended the Schmalkaldic War, a religious conflict between Protestants and Catholics within the Holy Roman Empire. The 1555 Peace of Augsburg tried to prevent their recurrence by fixing boundaries between the two faiths, using the principle of cuius regio, eius religio. This designated individual states as either Lutheran, then the most usual form of Protestantism, or Catholic, based on the religion of their ruler. Other provisions protected substantial religious minorities in cities like Donauwörth, and confirmed Lutheran ownership of property taken from the Catholic Church since Passau.

These agreements were undermined by the post-1555 expansion of Protestantism into areas previously designated as Catholic. Another factor was the growth of Protestant faiths not recognised by Augsburg, especially Calvinism, which was viewed with hostility by both Lutherans and Catholics. The Peace of Augsburg also gave individual rulers within the empire greater political autonomy and control over the religion practised in their domains, while weakening central authority. Conflict over economic and political objectives frequently superseded religion, with Lutheran Saxony, Denmark–Norway and Sweden competing with each other and Calvinist Brandenburg over the Baltic trade.

Managing these issues was hampered by the fragmented nature of the empire. Its representative institutions included 300 Imperial Estates distributed across Germany, the Low Countries, Northern Italy, and present-day France. These ranged in size and importance from the seven prince-electors who voted for the Holy Roman Emperor, down to prince-bishoprics and Imperial cities like Hamburg. Each also belonged to a regional grouping or " Imperial circle ", which primarily focused on defence and operated as autonomous bodies. Above all of these was the Imperial Diet, which only assembled on an irregular basis, and then largely served as a forum for discussion, rather than legislation.

Although, in theory, emperors were elected, the position had been held by the House of Habsburg since 1440. The largest single landowner within the Holy Roman Empire, they controlled lands containing over eight million subjects, including Austria, Bohemia and Hungary. The Habsburgs also ruled the Spanish Empire until 1556, when Charles V divided the two empires between different branches of the family. This bond was reinforced by frequent inter-marriage, while Spain retained Imperial territories such as the Spanish Netherlands, Milan and Franche-Comté. Although these links meant the Austrian and Spanish Habsburgs often worked together, their objectives did not always align. Spain was a global maritime superpower, whose possessions stretched from Europe to the Philippines, and much of the Americas. In contrast, Austria was a land-based power, focused on ensuring their pre-eminence within Germany and securing their eastern border against the Ottoman Empire.

Before Augsburg, unity of religion compensated for lack of strong central authority; once removed, it presented opportunities for those who sought to further weaken it. These included ambitious Imperial states like Lutheran Saxony and Catholic Bavaria, as well as France, confronted by Habsburg lands on its borders to the North, South, and along the Pyrenees. Since many foreign rulers were also Imperial princes, divisions within the empire drew in external powers like Christian IV of Denmark, who joined the war in 1625 as Duke of Holstein-Gottorp.
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