For more than forty years Hertzberg played an active part in the Prussian foreign office. In this capacity he had a decisive influence on Prussian policy, both under Frederick the Great and Frederick William II. At the beginning of the Seven Years' War (1756) he took part as a political writer in the Hohenzollern-Habsburg quarrel, both in his _Ursachen, die S.K.M. in Preussen bewogen haben, sich wider die Absichten des Wienerischen Hofes zu setzen und deren Ausfuhrung zuvorzukommen_ ("Motives which have induced the king of Prussia to oppose the intentions of the court of Vienna, and to prevent them from being carried into effect"), and in his _Memoire raisonne sur la conduite des cours de Vienne et de Saxe_, based on the secret papers taken by Frederick the Great from the archives of Dresden. After the defeat at Kolin (1757) he hastened to Pomerania in order to organize the national defence there and collect the necessary troops for the protection of the fortresses of Stettin and Colberg. In the same year he conducted the peace negotiations with Sweden, and was of great service in bringing about the peace of Hubertsburg (1763), on the conclusion of which the king received him with the words, "I congratulate you. You have made peace as I made war, one against many." In the later years, too, of Frederick the Great's reign, Hertzberg played a considerable part in foreign policy. In 1772, in a memoir based upon comprehensive historical studies, he defended the Prussian claims to certain provinces of Poland. He also took part successfully as a publicist in the negotiations concerning the question of the Bavarian succession (1778) and those of the peace of Teschen (1779). But in 1780 he failed to uphold Prussian interests at the election of the bishop of Munster. In 1784 appeared Hertzberg's memoir containing a thorough study of the _Furstenbund_. He championed this latest creation of Frederick the Great's mainly with a view to an energetic reform of the empire, though the idea of German unity was naturally still far from his mind. In 1785 followed "An explanation of the motives which have led the king of Prussia to propose to the other high estates of the empire an association for the maintenance of the system of the empire" (_Erklarung der Ursachen, welche S.M. in Preussen bewogen haben, ihren hohen Mitstanden des Reichs eine Association zur Erhaltung des Reichssystems anzutragen_). By upholding the Furstenbund Hertzberg made many enemies, prominent among whom was the king's brother, Prince Henry. Though the _Furstenbund_ failed to effect a reform of the empire, it at any rate prevented the fulfilment of Joseph II.'s old desire for the incorporation of Bavaria with Austria. The last act of state in which Hertzberg took part under Frederick the Great was the commercial treaty concluded in 1785 between Prussia and the United States.