Together with his uncle, Cornelis de Graeff, De Witt brought about peace with England after the First Anglo-Dutch War with the Treaty of Westminster in May 1654. The peace treaty had a secret annex, the Act of Seclusion, forbidding the Dutch ever to appoint William II's posthumous son, the infant William, as stadholder. This annex had been attached on instigation of Cromwell, who felt that since William III was a grandson of the executed Charles I, it was not in the interests of his own republican regime to see William ever gain political power.