led to conflict between Protestants and Catholics within the Holy Roman Empire, while the 1555 Peace of Augsburg tried to prevent their recurrence by fixing boundaries between the two faiths. Under the principle of cuius regio, eius religio, states were categorised 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, as well as the growth of Reformed faiths not recognised by Augsburg, especially Calvinism, a theology 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 Conflict over economic and political objectives frequently superseded religion, with Lutheran Saxony, Denmark-Norway and Sweden competing with each other and Calvinist Brandenburg over the Baltic trade. Reconciling these differences was hampered by the fragmented nature of the Empire and its representative institutions, which included 300 Imperial Estates distributed across Germany, the Low Countries, Northern Italy and modern France. They 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 them sat the Imperial Diet, which prior to 1663 assembled on an irregular basis and largely served as a forum for discussion, rather than legislation. Although Emperors were elected, since 1440 the position had been held by a member of the House of Habsburg, the largest single landowner in the Holy Roman Empire with territories containing over eight million subjects, including Austria, Bohemia and Hungary. They also ruled Spain until 1556 when the two empires were divided, although Spain retained Imperial states such as Milan and Franche-Comté. While the Austrian and Spanish Habsburgs often worked together, their objectives did not always align. The Spanish Empire was a global maritime superpower whose possessions included the Spanish Netherlands, Milan, the Kingdom of Naples, 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