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. Background: 1556 to 1618 Disputes occasionally resulted in full-scale conflict like the 1583 to 1588 Cologne War, caused when its ruler converted to Calvinism. More common were events such as the 1606 'Battle of the Flags' in Donauwörth, when riots broke out after the Lutheran majority blocked a Catholic religious procession. Emperor Rudolf approved intervention by the Catholic Maximilian of Bavaria. In return, he was allowed to annex the town and as agreed at Augsburg, the official religion changed from Lutheran to Catholic. When the Imperial Diet opened in February 1608, both Lutherans and Calvinists united to demand formal re-confirmation of the Augsburg settlement. However, in return the Habsburg heir Archduke Ferdinand required the immediate restoration of all property taken from the Catholic church since 1555, rather than the previous practice whereby the court ruled case by case. This threatened all Protestants, paralysed the Diet, and removed the perception of Imperial neutrality. Loss of faith in central authority meant towns and rulers began strengthening their fortifications and armies; outside travellers often commented on the growing militarisation of Germany in this period. This was taken a stage further in 1608 when Frederick IV, Elector Palatine formed the Protestant Union and Maximilian responded by setting up the Catholic League in July 1609. Both structures were primarily designed to support the dynastic ambitions of their leaders, but their creation combined with the 1609 to 1614 War of the Jülich Succession to increase tensions throughout the Empire. Some historians who see the war as primarily a European conflict argue Jülich marks its beginning, with Spain and Austria backing the Catholic candidate, France and the Dutch Republic the Protestant. External powers became involved in what was an internal German dispute due to the imminent expiry of the 1609 Twelve Years' Truce, which suspended the Eighty Years' War between Spain and the Dutch Republic. Before