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In 1975, the Prime Minister of Sikkim appealed to the
Indian Parliament for Sikkim to become a state of India. In April of that year, the
Indian Army took over the city of
Gangtok and disarmed the Chogyal's palace guards. Thereafter,
a referendum was held in which 97.5 per cent of voters supported abolishing the monarchy, effectively approving union with
India.
India is said to have stationed 20,000–40,000 troops in a country of only 200,000 during the referendum
https://en.wikipedia.org/wiki/Sikkim#Indian_protectorate_and_statehood
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Yes, Sikkim want to Voluntarily merged with India because 20,000–40,000 troops aim their gun against Sikkim people's head , if they don't "Voluntarily" , their head will blow up.
What a Voluntarily , good to know.