Peace of Augsburg
Politically, Frederick positioned himself as a leader of the Protestant princes in the Holy Roman Empire, and as a defender of the liberty of the German nobles against the Catholic Matthias, Holy Roman Emperor. Since the Peace of Augsburg, the Empire had been delicately balanced between Catholic, Lutheran, and Calvinist principalities (although Calvinism was not recognised in the Peace of Augsburg). The conflicts between princes of these three faiths developed into a deep struggle over the Empire's constitution. Furthermore, the Twelve Years' Truce, a hiatus in the Eighty Years' War, was set to expire in 1621, and was likely to lead to renewed fighting between the Dutch Republic and the Spanish Empire.