Sikkim was being ruled for more than 300 years by Chogyals. In 1947 when India became independent, a popular vote in Sikkim rejected joining Indian Union. However, Chogyal Tashi Namgyal was successful in getting a special status of protectorate for Sikkim. This was in face of stiff resistance from local parties like Sikkim State Congress who wanted a democratic setup and accession of Sikkim to the Union of India
The treaty signed between India and Sikkim ratified the status of Sikkim as a protectorate with Chogyal as the Monarch. Chogyal Tashi Namgyal died in 1963 and was succeeded by his son Palden Thondup Namgyal. By the beginning of 1970 there were rumbling in the political ranks and file of the State, which demanded the removal of Monarchy and the establishment of a democratic setup. This finally culminated in wide spread agitation against Sikkim Durbar in 1973.
On 8th May, 1973, there was a historic agreement between the Chogyal, the leaders of the political parties representing the people of Sikkim and the Government of India.
The Government of Sikkim Bill was passed on May 11, 1974. Chogyal promulgated this Bill on the 4th July, 1974 as the Government of Sikkim Act, 1974
By Constitution 35th Amendment Bill on 22nd February, 1975 a Tenth Schedule was added in the constitution. This amendment act was called Constitution (Thirty-fifth Amendment) Act, 1974.
Also, a new article 2A was inserted after article 2 of the constitution of India.
Article 2A read as follows:
Sikkim to be associated with the Union
Part A of The Tenth Schedule read as follows:
TERRITORIES OF SIKKIM
1. Sikkim. ---Sikkim comprises the following territories, namely:-
The territories which, immediately before the coming into force of the Government of Sikkim Act, 1974, were comprised in Sikkim.
Accordingly Government of India became solely responsible for the defense and territorial integrity of Sikkim and for the conduct and regulation of the external relations of Sikkim, whether political, economic or financial.
However, there was a complete collapse in the administration in Sikkim. The Chogyal was proving to be extremely unpopular with the people. In 1975, the Kazi (Prime Minister in Sikkim) appealed to the Indian Parliament for a change in Sikkim's status so that it could become a state of India.
In April, the Indian Army took over the city of Gangtok and disarmed the Palace Guards. A referendum was held in which 97.5% of the voting people (59% of the people entitled to vote) voted to join the Indian Union. A few weeks later, on 16 May 1975, Sikkim officially became the 22nd state of the Indian Union through Constitution 36th Amendment Act and the monarchy was abolished.
Constitution 36th Amendment act was passed on 16th May, 1975. Through this act First Schedule (which lists the states and territories on of India, lists any changes to their borders and the laws used to make that change) was amended and Sikkim was entered on entry 22.
A new article 371 F was also added in the Constitution which provided for some special provisions with respect to State of Sikkim.
In the same act, 4th schedule was also amended and after entry 21, Sikkim was added at entry 22.
The Article 2A, which was added by 35th amendment act was repealed by this 36th amendment act and Schedule 10 was also omitted.
Since 10th Schedule was omitted as Sikkim became India's fully fledged state, the Constitution (Fifty-second Amendment) Bill, 1985 again added a New 10th Schedule which was related to Anti Defection Law.