On 7 June 1821, he was transferred to the
43rd Monmouthshire light infantry, in which he was promoted lieutenant on 9 December 1824, and captain on 4 November 1825. He spent the next five years of his military life in
Gibraltar. He returned to England and then Ireland. In 1834 he accepted the post of second in command to Colonel
F. R. Chesney in the famous
Euphrates Valley Expedition, and was placed in charge of the magnetic experiments. He showed himself a loyal assistant to his chief during the next two years of arduous labour and travel, and it was chiefly owing to Chesney's advocacy of his services that Estcourt was promoted major on 21 October 1836, and lieutenant-colonel by brevet on 29 March 1839. His regiment participated in the suppression of the
Lower Canada Rebellion, and he was based eventually in
Drummondville, Upper Canada, where, in addition to other activities as surveyor, he brought the attention of his superiors to the poor condition of the Cayuga Road. ==Treaty of Webster Ashburton==