He did very great service in furthering Pitt's policy of maintaining England's influence on the Continent by the arms of her allies, and held the threads of the diplomacy which ended in the king of Prussia's overthrowing the Patriot republican party in the Dutch Republic, which was inclined to France, and re-establishing the stadtholder William V, Prince of Orange in his dictatorial powers. As envoy, Harris immersed himself in Dutch politics from 1784 on and managed to become the "de facto" leader of the Orangist party. He and his French counterpart, Charles Olivier de Saint-Georges de Vérac, the French ambassador to the States General of the Netherlands, fought a secret war with the help of agents of influence, like the then-Grand Pensionary of the province of Zeeland, Laurens Pieter van de Spiegel, and the confidential agent Hendrik August van Kinckel, and spies like Pierre Auguste Brahain Ducange. Harris returned to London in secret at the end of May 1787, where he managed to convince the Cabinet to endorse a policy of subversion in the Dutch Republic, to be funded by £70,000 from a slush fund, laundered through the king's Civil list. Harris agents used the money to bribe regiments of the Dutch States Army in the pay of the Patriot States of Holland, that had deposed the stadtholder as Captain-General of that Army, to defect. The counter-measures of the States of Holland precipitated a political crisis that prompted the States to ask for French mediation. The arrest of Princess Wilhelmina, the wife of the stadtholder, on 28 June 1782, gave Prussia and Great Britain an opening to muscle in on this diplomatic mediation, and eventually offered an excuse to intervene militarily. In recognition of his services he was created Baron Malmesbury, of Malmesbury in the County of Wiltshire on 19 September 1788, and permitted by the King of Prussia to bear the Prussian eagle on his arms, and by the Prince of Orange to use his motto "Je maintiendrai".