On 21 March 1825, Cappellari was created cardinal "in pectore" (published 13 March 1826) by Pope Leo XII, and shortly afterwards he was asked to negotiate a concordat to safeguard the rights of Catholics in the Low Countries, a diplomatic task which he completed successfully. He also negotiated a peace on behalf of Armenian Catholics with the Ottoman Empire. He publicly condemned the Polish revolutionaries, who he thought were seeking to undermine Russian Tsar Nicholas I's efforts to support the Catholic royalist cause in France by forcing him to divert his troops to suppress the uprising in Poland.