In 1782 he entered the service of Charles William Ferdinand, Duke of Brunswick, and as president of the board of domains displayed a zeal for reform, in the manner approved by the enlightened despots of the century, which rendered him very unpopular with the orthodox clergy and the conservative estates.

The position, owing to the singular overlapping of territorial claims in the old Holy Roman Empire, was one of considerable delicacy, and Hardenberg filled it with great skill, doing much to reform traditional anomalies and to develop the country, and at the same time labouring to expand the influence of Prussia in southern Germany.

[2] In 1797, on the accession of King Frederick William III of Prussia, Hardenberg was summoned to Berlin, where he received an important position in the cabinet and was appointed chief of the departments of Magdeburg and Halberstadt, for Westphalia, and for the Principality of Neuchâtel.

When Haugwitz had returned, the unyielding attitude of Napoleon had caused the king to make advances to Russia, but the mutual declarations of 3 and 25 May 1804 pledged both powers to take up arms only in the event of a French attack upon Prussia or of further aggressions in northern Germany.

The Battle of Jena–Auerstedt and its consequences had had a profound effect upon him, and in his mind, the traditions of the old diplomacy had given place to the new sentiment of nationality characteristic of the coming age, which in him found expression in a passionate desire to restore the position of Prussia and crush her oppressors.

[2] During his retirement at Riga, he had worked out an elaborate plan for reconstructing the monarchy on liberal lines, and when he came into power, the circumstances of the time did not admit of his pursuing an independent foreign policy, but he steadily prepared for the struggle with France by carrying out Stein's far-reaching schemes of social and political reorganization.

[2] The military system was completely reformed, serfdom was abolished, municipal institutions were fostered, the civil service was thrown open to all classes and great attention was devoted to the educational needs of every section of the community.

When at last the time came to put the reforms to the test, after the French invasion of Russia in 1812, it was Hardenberg who persuaded Frederick William to take advantage of Ludwig Yorck von Wartenburg's loyal disloyalty and to declare against France.

He was rightly regarded by German patriots as the statesman who had done most to encourage the spirit of national independence, and immediately after he had signed the first Peace of Paris in 1814, he was raised to the rank of prince 3 June 1814 in recognition of the part he had played in the War of the Sixth Coalition.

In the Second Treaty of Paris, after the Battle of Waterloo, he failed to carry through his views as to the further dismemberment of France and had weakly allowed Metternich to forestall him in making terms with the states of the Confederation of the Rhine, which secured to Austria the preponderance in the German federal diet.

On the eve of the conference of Carlsbad (1819) he signed a convention with Metternich in which, according the historian Heinrich von Treitschke, 'like a penitent sinner, without any formal quid pro quo, the monarchy of Frederick the Great yielded to a foreign power a voice in her internal affairs.

In the privacy of royal commissions, he continued to elaborate schemes for constitutions that never saw the light, but Germany, disillusioned, regarded him as an adherent of Metternich, an accomplice in the policy of the Carlsbad Decrees and the Troppau Protocol.