Sforza found out how this change had come about, and learned that it was Piero's influence that had overmastered his own.

He could not disentangle the real motives that had promised the change, and imagined there was some secret league against himself: he attributed the changed political programme to the death of Lorenzo dei Medici.

But whatever its cause might be, it was evidently prejudicial to his own interests: Florence, Milan's old ally, was abandoning her for Naples.

He resolved to throw a counter weight into the scales; so, betraying to Alexander the policy of Piero and Ferdinand, he proposed to form a defensive and offensive alliance with him and admit the republic of Venice; Duke Hercules III of Ferrara was also to be summoned to pronounce for one or other of the two leagues.

Alexander VI, wounded by Ferdinand's treatment of himself, accepted Ludovico Sforza's proposition, and an Act of Confederation was signed on the 22nd of April, 1493, by which the new allies pledged themselves to set on foot for the maintenance of the public peace an army of 20,000 horse and 6,000 infantry.
Ferdinand was frightened when he beheld the formation of this league; but he thought he could neutralise its effects by depriving Ludovico Sforza of his regency, which he had already kept beyond the proper time, though as yet he was not strictly an usurper.