The final struggle was to come at the vacancy of the Imperial throne. Ferdinand of Aragon was dead, and Naples passed to the King of undivided Spain. It was the unswerving policy of Rome that it should not be united with the Empire, and against that fixed axiom the strongest dynasty of emperors went to pieces. The Reformation had just begun in Germany, and Leo wished one of the Northern Electors to be chosen as Maximilian's successor. In conformity with the political situation, he would have preferred Frederic of Saxony, the protector of Luther. The election of Charles, in 1519, was a defiance of the Balance of Power, a thing not to the taste of the Middle Ages, but becoming familiar in those days. France, unable formerly to keep Naples against Spain, had now to defend Lombardy against Spain, supported by Germany, Naples, and the Netherlands. Francis maintained the unequal struggle for four years, although his most powerful vassal, Bourbon, brought the enemy to the gates of Marseilles. The decisive action of the long Italian war was fought at Pavia in June 1525, where Francis was taken prisoner, and was compelled to purchase his release by cruel sacrifices. The years that followed are only a phase in the permanent subjugation of Italy, but they are memorable in another connection. For the triumph of Pavia brought the suppression of the Lutherans within the range of practical politics. The Peasants' War had damaged their position; the Emperor was able now to execute the Imperial decree of Worms, and there were some in Germany who desired it. He made it a condition of his prisoner's deliverance that he should assist in destroying them; and Francis readily offered to do it by coming in person, and bearing half the charge. Charles proposed to take him at his word, when he learnt that the Pope was at the head of a great alliance against him. Pope Clement was advised by the best ecclesiastic in his court, the Datario Giberti, to try one more struggle before the chains were riveted, and before he became, as they said, a Spanish chaplain. It is a war, said Giberti, not for power or dominion, but for the redemption of Italy from perpetual bondage; and he placed his master, for the moment, at the head of the nation. Clement concluded a treaty with the Emperor's enemies at Cognac, released Francis from his oath to observe the Treaty of Madrid, and endeavoured to make Pescara, the victor of Pavia, turn traitor by the prospect of the throne of Naples.