The acceptance of the "Interim" was resisted both by Catholics and Protestants. Charles V. used all his arts,--persuasion, threats, armed force,--and succeeded for a short time in compelling a sort of external observance of its provisions. His ambition, now, was to have his son Philip chosen by the Diet as his successor, notwithstanding that Ferdinand of Austria had been elected king in 1530, and had governed during his brother's long absence from Germany. The Protestant Electors, conquered as they were, and abject as many of them had seemed, were not ready to comply; Ferdinand's jealousy was aroused, and the question was in suspense when a sudden and startling event changed the whole face of affairs. Maurice of Saxony had been besieging Magdeburg for a year, in the Emperor's name. The city was well-provisioned, admirably defended, and the people answered every threat with defiance and ridicule. Maurice grew tired of his inglorious position, sensitive to the name of "Traitor" which was everywhere hurled against him, and indignant at the continued imprisonment of Philip of Hesse. He made a secret treaty with Henry II. of France, to whom he promised Lorraine, including the cities of Toul, Verdun and Metz, in return for his assistance; and then, in the spring of 1552, before his plans could be divined, marched with all speed against the Emperor, who was holding his court in Innsbruck. The latter attempted to escape to Flanders, but Maurice had already seized the mountain-passes. Nothing but speedy flight across the Alps, in night and storm, attended only by a few followers, saved Charles V. from capture. The Council of Trent broke up and fled in terror; John Frederick of Saxony and Philip of Hesse were freed from their long confinement, and the Protestant cause gained at one blow all the ground it had lost. [Sidenote: 1553. ALBERT OF BRANDENBURG'S RAID.] Maurice returned to Passau, on the Danube, where Ferdinand of Austria united with him in calling a Diet of the German Electors. The latter, bishops as well as princes, admitted that the Protestants could be no longer suppressed by force, and agreed to establish a religious peace, independent of any action of the Pope and Council. The "Treaty of Passau," as it was called, allowed freedom of worship to all who accepted the Augsburg Confession, and postponed other questions to the decision of a German Diet. The Emperor at first refused to subscribe to the treaty, but when Maurice began to renew hostilities, there was no other course left. The French in Lorraine and the Turks in Hungary were making rapid advances, and it was no time to assert his lost despotism over the Empire.