In , when a subaltern in the horse artillery, he accompanied Sir John Malcolm's mission to Persia as one of the officers of the escort. His tall stature — he was six feet eight inches in height without his shoes — is said to have greatly excited the admiration and curiosity of the Persians.
It is related of him that on one occasion, while the mission was in Persia, Sir John Malcolm overheard a Persian call out to one of Bethune's servants, 'Is your date-tree asleep or awake? He returned to England in , retiring in the following year from the service of the East India Company, and settling in Scotland on the estate of Kilconquhar, to which he had succeeded on the death of his grandifather.
On succeeding to the estate he adopted the surname of Bethune, in conformity with the deed of entail. In he was sent back to Persia by the British government, and commanded a part of the Persian army in the war of succession in the following year, leading his division from Tabriz to Teheran, and completely quelling the rebellion against Mahomed Shah, the successor of the late Shah, Fath-i-Ali Khan.
For this service he received from the Shah the order of the Lion and Sun, and on his return to England was created a baronet, in accordance with a special request made by the Shah, that his majesty would confer upon Bethune 'some rank which in the English state may descend lineally to his posterity, and always remain in his family. In Bethune was a third time sent to Persia, with the local rank of major-general in Asia, to take command of the Persian army; but owing to a misunderstanding of, arising from the Persian advance upon Herat, the Shah's government declined to allow him to take up this command.
He accordingly returned to England in , and finally retired from military life. Some years afterwards he again visited Persia as a traveller, and died at Tabriz in Sir Henry Bethune married in a daughter of John Trotter, of Dyrham Park, Hertfordshire, by whom he had three sons and five daughters.
