Tempted however as he must have been to a new turn in his policy by the failure of his older plans at home and the display of French greatness, the sudden and decisive turn which he actually gave it was due above all to an event which, unknown as it as yet remained to Englishmen, was destined to exercise a vast influence from this moment on English politics. This was the conversion of his brother and presumptive successor James, Duke of York, to the Catholic faith. Though finally completed in the spring of 1672, this had for some time been imminent. The dull, truthful temper of the Duke hindered him from listening to his brother's remonstrances against this step; but Charles was far too keen-witted to be blind to the difficulties in which it was certain to involve him. That either Churchman or Presbyterian should sit still and wait patiently the advent of a Catholic king, and above all a king whose temper would necessarily make him a Catholic bigot, was, as he foresaw, impossible. The step could not long be concealed; and when once it was known a demand would arise for the exclusion of James from the succession, or at the least for securities which would fetter the Crown. Even if such a demand were surmounted a struggle between James and the Parliament was in the end inevitable, and such a struggle, if it ever arose, could end only in the establishment of Catholicism and despotism or in the expulsion of James from the throne. To foresee these consequences required no great keenness of sight; they were as plainly foreseen by Ashley and the bulk of Englishmen, when once the truth was known, as by Charles. But Charles was far from contenting himself with foreseeing them. He resolved to anticipate the danger by hurrying on the struggle which was certain to come. France alone could help him in forcing despotism and Catholicism on England, and from this moment Charles surrendered himself utterly to France. He declared to Lewis his purpose of entering into an alliance with him, offensive and defensive. He owned to being the only man in his kingdom who desired such a league, but he was determined, he said, to realize his desire, whatever might be the sentiments of his ministers. [Sidenote: Treaty of Dover.]