In July 1824 he enlisted as a surgeon with the East India Company's army. "That he had never actually studied medicine was not, at least in his own mind, an impediment." The Company was about to enter a war in Burma and needed qualified surgeons. Relying on self-study and some practice while at sea, Harlan presented himself to the medical board for examination and was appointed as surgeon to the Calcutta general hospital. Beginning in January 1825 he served with the army in Burma. Harlan admired the impressive capacity of the Company's sepoys who "consumed nothing but parched grain, a leguminous seed resembling the pea", and yet kept going. Owing to heavy losses by Company troops due to disease and war, Harlan sometimes fought with the Bengal Artillery, acquiring military knowledge that was later to serve him well. Harlan was at the Battle of Prome, where Anglo-Indian forces stormed the city of Prome (modern Pyay) and engaged in fierce hand-to-hand fighting with the Burmese. The Treaty of Yandabo in 1826 ended hostilities.