He at once applied himself with exemplary diligence to the study of the Arabic and Persian languages, and of the religions and customs of India.

Passing in due course through the ordinary early stages of military life, he was promoted to the rank of ensign on the 23rd September, 1810, and to that of lieutenant on the 16th December, 1814.
Lieutenant Sleeman served in the war with Nepal, which began in 1814 and terminated in 1816.

During the campaign he narrowly escaped death from a violent epidemic fever, which nearly destroyed his regiment.
'Three hundred of my own regiment,' he observes, 'consisting of about seven hundred, were obliged to be sent to their homes on sick leave.
The greater number of those who remained continued to suffer, and a great many died.

Of about ten European officers present with my regiment, seven had the fever and five died of it, almost all in a state of delirium.

I was myself one of the two who survived, and I was for many days delirious.[1] The services of Lieutenant Sleeman during the war attracted attention, and accordingly, in 1816, he was selected to report on certain claims to prize-money.