Meanwhile, the religious question in Germany had all along been considered by Charles as one of his most important problems. The first war with France prevented any action on his part until 1529, since he needed the support of the Protestant princes. The movement therefore had time to gather headway, and it was evident that Charles would meet with determined opposition whenever he should decide to face the issue. Various factors entered in to complicate the matter, such, for example, as the fear on the part of many princes of the growing Hapsburg power and the belief that Charles meant to make the imperial succession hereditary in his family. A temporary adjustment of the religious situation was made by the imperial Diet held at Spires in 1526, when it was agreed that every prince should decide for himself in matters of religion. With the close of the war with France in 1529, Charles caused the Diet to meet again at Spires, on which occasion the previous decision was revoked. The princes devoted to the reform ideas protested, giving rise to the name “Protestant,” but without avail. The Diet was called for the next year at Augsburg, when Charles sat in judgment between the two parties. The Protestants presented their side in a document which became known as the confession of Augsburg. The Catholic theologians replied, and Charles accepted their view, ordering the Protestant leaders to submit, and threatening to employ force unless they should do so. The international situation again operated to protect the reform movement, for the Turks became threatening, and, indeed, what with the wars with France and his numerous other difficulties Charles was unable to proceed resolutely to a solution of the religious problem until the year 1545. At last he was ready to declare war. In 1547 he won what seemed to be a decisive victory in the Battle of Mühlberg, resulting in the subjection of the Protestant princes to the Roman Church. They protested anew, and, aided by the opposition to Charles on other grounds,--for example, because of his introduction of Italian and Spanish soldiery into what was regarded as a domestic quarrel,--were able to present a warlike front again. This time they were joined by Charles’ former powerful ally, Maurice of Saxony, through whose assistance they successfully defended themselves. Peace was made at Passau in 1552, ratified by the Diet of Augsburg in 1555, whereby the Protestant princes obtained equal rights with the Catholic lords as to their freedom in religious beliefs. [Sidenote: Other failures of Charles and his abdication.]