PROTESTANT UNION. – A defensive league formed in Bavarian Swabia on 16 May 1608 between the representative of the Palatine Elector Christian of Anhalt, the Duke of Württemberg, the Margraves of Ansbach, Kulmbach, Baden-Durlach, and Neuburg, initially for ten years, later extended in 1617 for a further three. It was subsequently joined in 1609 by the free cities of Strasbourg, Ulm, and Nuremberg; in 1610 by the Elector of Brandenburg, the Margrave of Hesse-Kassel, and numerous other cities.
Under the presidency of the Palatine Elector Frederick IV, the Union immediately sought to conclude secret agreements with foreign princes hostile to the House of Habsburg, who were interested in preventing the strengthening of the unitary principle in Germany, i.e., supporters of the Protestant cause. It thus succeeded in negotiating with France, though these talks suffered a setback with the death of Henry IV; it established formal agreements with England (1612) and with the Netherlands (1613); and it at least secured the benevolent support of Sweden and Denmark. The occasion that gave rise to the Union, to which the Catholic side responded as early as 1609 with the Catholic League, was a dispute that arose in Donauwörth over the interpretation of the Peace of Augsburg of 1555. In reality, the Protestants intended to form a united front against the Catholic revival in religious and political matters, which threatened their more or less regular possession of selected territories and goods and sought to exploit the doctrinal and political-territorial differences between Calvinists and Lutherans. Thus, it became the center of all anti-Catholic and anti-Habsburg intrigues and, as such, intervened in the question of the succession in the Duchy of Jülich in favor of the Protestant claimant (1609–10) and helped create the atmosphere that led to the Bohemian uprising, which inaugurated the politico-religious Thirty Years’ War. After the defeat of the insurgents at the Battle of White Mountain, following an initial truce with the Catholic League, the Union dissolved in May 1621, only to re-emerge later in a different form, prompted by Richelieu’s anti-Habsburg policy.