All these duties and callings he relinquished in the summer of 1827, when he entered upon the diplomatic service, which opened before him a new career of usefulness. It was then that he became Chargé d'Affaires at Copenhagen, where he continued till 1835, when he was transferred by President Jackson to Berlin, as Minister Resident. In 1837 he was raised by President Van Buren to the rank of Envoy Extraordinary and Minister Plenipotentiary at the same court. On July 22, 1846, he had his audience of farewell from the King of Prussia, being recalled by President Polk. This long term of service was passed abroad with the intermission of a brief period in 1834, when he revisited his country on leave of absence. During this protracted career in foreign countries, charged with responsible negotiations, he was not lost in the toils of office, or in the allurements of court life. He was always a student. At Copenhagen he prepared his "History of the Northmen, or Danes and Normans, from the Earliest Times to the Conquest of England by William of Normandy." This was published in 1831, both in England and in America. In 1844 it was much enlarged, and translated into French. At the time of his death he was occupied in preparing another edition. In 1838 he contributed to the Edinburgh Cabinet Library a portion of the volumes entitled "Scandinavia." By these works he earned an honorable place among our historical writers. His History of the Northmen preceded, in time, the productions of Bancroft and Prescott, which have since achieved so much renown.