Context: 

Sikkim day is annually celebrated on May 16, recalling the history of the former kingdom’s integration with India in 1975.

About Sikkim’s History:

• The kingdom of Sikkim was established in 1642 by the Chogyal dynasty.
• Sandwiched between India and China, and often party to conflicts over land with Bhutan and Nepal, the British colonisation of India first led to a kind of formal relationship developing between the two states.
• The British saw Sikkim as a buffer state against China and Nepal, with whom they fought in the Anglo-Gorkha war of 1814-16, helping Sikkim secure a number of territories that Nepal had previously captured.
• A formal protectorate was established over Sikkim through the Treaty of Tumlong in 1861, meaning the British had control over it but it was not officially under their rule and the Chogyals could continue holding onto power.

Integration with India:

• Anti-monarchy protests grew in Sikkim in 1973, following which the royal palace was surrounded by thousands of protesters. 
• Indian troops arrived after the monarch asked New Delhi to send assistance. 
• A tripartite agreement was signed in 1973 between the chogyal, the Indian government, and three major political parties, so that major political reforms could be introduced.
• A referendum was held in Sikkim in 1975 in which majority votes were cast in favour of abolishing the monarchy and joining India.
• India’s Ministry of External Affairs introduced the Constitution (Thirty-Sixth Amendment) Bill in the Lok Sabha to recognise Sikkim as a state in the Union of India. This was passed in the Parliament and assented to by the President in 1975.                                                                                                                                                     News Source: Indian Express