Hystaspes on the rock-face at Behistun, near Kirmânshah. It was partly with a view to prosecuting his researches there that he accepted, in 1843, the post of political agent of the East India Company in Turkish Arabia, to which was added that of consul at Baghdad on 5 March 1844, a post which had been held by a series of distinguished scholars and soldiers, and which was important alike politically and archæologically. The voluminous but as yet unpublished correspondence which Rawlinson carried on with the ambassador at the Porte, Sir Stratford Canning [q. v.], contains abundant proof of the ability displayed by the consul at Baghdad in watching over British interests on the Turco-Persian frontier. That the government appreciated his vigilance is shown by their raising him to the rank of consul-general on 22 Nov. 1851.
But side by side with his official duties the fascination of cuneiform research absorbed the balance of his vigorous energies. He had begun to copy the undeciphered Behistun inscription as early as 1835, and the task was resumed with renewed enthusiasm on his return as consul at Baghdad. A large part of 1844–5 was devoted to the great inscription, and at last, in 1846, at considerable personal risk, and after no trifling exercise of patience and endurance, the complete copy was finished and the decipherment carried to a triumphant conclusion. Rawlinson sent home a full text, translation, and notes of ‘The Persian Cuneiform Inscription at Behistun,’ which was printed, with numerous plates, in the ‘Journal of the Royal Asiatic Society,’ in 2 vols., 1846 (Appendices, 1850 and 1853). By a singular coincidence, Dr. Edward Hincks [q. v.] of Killyleagh, co. Down, had simultaneously, and quite independently, arrived at similar philological results by his signal discovery of the Persian cuneiform vowel system, which he published in vol. xxi. of the ‘Transactions of the Royal Irish Academy.’ The accuracy of the new decipherment was afterwards tested by submitting an undeciphered inscription of Tiglath Pileser I separately to Rawlinson, Hincks, Oppert, and Fox Talbot, whose independent translations, on examination by a mixed committee, including Horace Hayman Wilson, William Cureton, Sir Gardner Wilkinson, Whewell, Milman, and Grote, were found to resemble each other so closely that no further doubt could be entertained. The importance of the discovery for philology and ancient history is only paralleled by Young and Champollion's decipherment of the Rosetta stone, and it is natural that there should be some competition for priority in so momentous a discovery. Many scholars, from Grotefend downwards, and notably Hincks, contributed towards the elucidation of the problem of cuneiform discovery; but, while their claims and merits must not be undervalued, it is indisputable that, at least so far as the decipherment of the Persian class of cuneiform writing is concerned, Rawlinson's accurate transcription of the Behistun inscription, with his scholarly interpretation of the text, is the most important contribution to the subject; and his claim to be the first successful decipherer of cuneiform was soon admitted in Germany. Dr. Oppert said well of him: ‘Rawlinson était un homme d'un génie prime-sautier, et ce qui est encore plus rare, il avait le don de tomber juste’ (Cordier, Éloge, Soc. de Géogr. de Paris, 1895). As a general Assyriologist, as a philologist and man of learning, he has been surpassed by others; as a discoverer and bold instinctive interpreter of an undeciphered language, perhaps by none.
Rawlinson returned to England in 1849. The signal importance of his discovery was recognised on all hands, and inspired further research. The trustees of the British Museum made him a grant of 3,000l. for excavations in Babylonia, and by his energy and skill many valuable sculptures were added to the museum collections. Rawlinson resigned his consulship on 19 Feb. 1855, and, returning home, was made a K.C.B. on 4 Feb. 1856. He received the rank of honorary lieutenant-colonel on 25 March, and was appointed a crown director of the East India Company in the same year. In 1857 he unsuccessfully contested the representation in parliament of Reigate as a conservative, but on a second contest was returned on 4 Feb. 1858 to the House of Commons, where he spoke frequently on eastern questions, especially on the transfer of India from the company to the crown; and on 12 Sept. 1858 became one of the first members of the newly created India council, resigning at the same time his seat in parliament. He left the council in 1859, however, on being appointed, on 16 April, minister-plenipotentiary to Persia, with the army rank of major-general; but it soon appeared that the legation at Tehrân offered little attraction to a man of his political insight and pronounced views on Russian aggression. He resigned in less than a year, on 20 Feb. 1860, not, however, before he had established friendly personal relations with the shah. He again sat in the House of Commons for three years, for Frome, from August 1865 to 1868, and took the lead in advocating a vigorous anti-Russian policy in