As the Baloch Regiment, Sen's parent regiment, was among those regiments allotted to Pakistan following Indian independence, Sen transferred to the 8th Gorkha Rifles. During the Indo-Pakistani War of 1947–1948, he was promoted to acting brigadier and commanded the 161st Indian Infantry Brigade, receiving a mention in dispatches. On 16 March 1955, Sen was promoted acting major-general and appointed Director of Military Training (DMT). He was appointed Master-General of the Ordnance (MGO) on 8 May 1957, and was further appointed Colonel-Commandant of the Army Physical Training Corps on 26 September.