On 1 January 1812 he was promoted to the full rank of general, and a few months later he proceeded to Russia as ambassador and military commissioner. In the latter capacity he served with the headquarters of the allies throughout the War of Liberation (1812–1814); his success in the delicate and difficult task of maintaining harmony and devotion to the common cause amongst the generals of many nationalities was recognized after the war by his elevation to an earldom (July 1814). He then went to St. Petersburg, and continued to hold the post of ambassador until 1820, when he returned to the United Kingdom.