The great fabric of power which had been built up by the policy of Ferdinand of Aragon was thus threatened with utter ruin, and Charles saw himself forced into the struggle he had so long avoided, if not for the interests of religion, at any rate for the interests of the House of Austria. He still hoped for a reunion from the Council which was assembling at Trent, and from which a purified Catholicism was to come. But he no longer hoped that the Lutherans would yield to the mere voice of the Council. They would yield only to force, and the first step in such a process of compulsion must be the breaking up of their League of Schmalkald. Only France could save them; and it was to isolate them from France that Charles availed himself of the terror his march on Paris had caused, and concluded a treaty with that power in September 1544. The progress of Protestantism had startled even France itself; and her old policy seemed to be abandoned in her promises of co-operation in the task of repressing heresy in the Empire. But a stronger security against French intervention lay in the unscrupulous dexterity with which, while withdrawing from the struggle, Charles left Henry and Francis still at strife. Henry would not cede Boulogne, and Francis saw no means of forcing him to a peace save by a threat of invasion. While an army closed round Boulogne, and a squadron carried troops to Scotland, a hundred and fifty French ships were gathered in the Channel and crossed in the summer of 1545 to the Isle of Wight. But their attacks were feebly conducted and the fleet at last returned to its harbours without striking any serious blow, while the siege of Boulogne dragged idly on through the year. Both kings however drew to peace. In spite of the treaty of Crepy it was impossible for France to abandon the Lutherans, and Francis was eager to free his hands for action across the Rhine. Henry on the other hand, deserted by his ally and with a treasury ruined by the cost of the war, was ready at last to surrender his gains in it. In June 1546 a peace was concluded by which England engaged to surrender Boulogne on payment of a heavy ransom, and France to restore the annual subsidy which had been promised in 1525. [Sidenote: The Peace.]