In 1889, the Ao region was fully annexed to the Naga Hills District as a subdivision.

The Government of India Act 1919 declared the Naga Hills District as a "Backward Tract".

[2] In the early 1930s, some of the ethnic groups in the region rebelled against the British rule under the leadership of Haipou Jadonang and Rani Gaidinliu.

[3] Subsequently, the Naga Hills District, along with the Tuensang Division (then a "Part B" area in the North-East Frontier Agency) were made a new administrative unit under the Ministry of External Affairs in 1957.

[3] After negotiation with the secessionists, this administrative unit was later made a full-fledged state called Nagaland.