In 1914, some tribal-majority areas were separated from the former Darrang and Lakhimpur districts of Assam Province of British India to form the North-East Frontier Tract. The NEFT was initially divided into two sections: the Central & Eastern Section and the Western Section. Each section was placed under a political officer. In 1919, the Central and Eastern Section was renamed the Sadiya Frontier Tract, and the Western Section was renamed the Balipara Frontier Tract. In 1937, Sadiya and Balipara Frontier Tracts along with Lakhimpur Frontier Tract of Assam Province came to be known collectively as the 'Excluded Areas of province of Assam' under the provisions of Government of India Act, 1935. By Regulation 1 of The North Eastern Frontier Tracts Regulations 1943, Tirap Frontier Tract was created by amalgamating certain areas of Sadiya and Lakhimpur Frontier Tracts. After this a new regulation – The Assam Frontier Regulation, 1945 – was made by the Governor of Assam using his powers under Section 92 of the Government of India Act, 1935. This proved to be a landmark step in the history of NEFA which later emerged as the state of Arunachal Pradesh. In 1946, Balipara Frontier Tract was divided into two administrative units: Sela Sub-Agency and Subansiri Area. After the independence of India in 1947, NEFT became a part of Assam state. In 1948, Sadiya Frontier Tract was bifurcated into two districts: Abor Hills district and Mishmi Hills district. In 1950, the plain portions of these tracts, were transferred to the Assam state government and the rest became one of the Tribal Areas in Assam state. In 1951, Balipara Frontier Tract, Tirap Frontier Tract, Abor Hills district, Mishmi Hills district and the Naga tribal areas were together renamed as the North-East Frontier Agency. On 26 January 1954 the North-East Frontier Agency was divided into six frontier divisions: Kameng, Subansiri, Tirap, Siang, Lohit and Tuensang. On 1 December 1957, Tuensang was separated and attached to the newly formed Naga Hills district to form Naga Hills-Tuensang Area. On 1 August 1965 the administration of the agency was transferred from the Ministry of External Affairs to the Ministry of Home Affairs. Consequently, on 1 December 1965, the five frontier divisions became its five districts. A deputy commissioner became the administrative head of these districts in place of a political officer. In 1967, an Agency Council was constituted for better administration. Until 1972, it was constitutionally a part of Assam state and was directly administered by the governor of Assam as agent of the President of India. On 21 January 1972, the North-East Frontier Agency became the Union Territory of India and was placed under the charge of a Chief Commissioner.
