Later, in December 1972, when the Chogyal (king) of Sikkim was pressuring India to revise the Indo-Sikkim Treaty, the Prime Minister of India,
Indira Gandhi, called Kao and asked him to do something about Sikkim. Banerjee, under whose purview the Sikkim operations came, prepared a strategy within a fortnight, which was instantly cleared by Mrs. Gandhi and ultimately culminated into merger of Sikkim into India. Banerjee and Ajit Singh Syali (then OSD in Gangtok and later, chief of R&AW) launched an operation codenamed
Janamat and Twilight, by which they undermined the monarchy by strengthening the agitations of the Sikkim National Congress, led by
Kazi Lhendup Dorjee. By 8 April 1973, the Chogyal was forced to sign a draft treaty with India, which stated that the administration would be taken over by India. ==Death==