Is that what they taught you in your school textbook.
Well, you need to study a bit more. Here's a start:
"India openly supported the movement against King (Chogyal) Palden Thondup Namgyal. The then ADC to the King, Captain Sonam Yongda, claimed that soldiers of Indian Army in civil dress used to take part in the protests. Some of the protesters were brought from Darjeeling and the surrounding areas. The number of Sikkimese who took part in the protest was quite small. But that was enough.
Lhendup’s protest movement depended mainly on Indian financial assistance. The money was made available through Intelligence Bureau (IB). “The people from IB used to visit me twice or thrice a year. An IB agent, Tejpal Sen, used to handover money to me personally”, Dorji had told me in a recorded interview.
In fact, the main actor behind the ‘Mission Sikkim’ was India’s external intelligence agency, RAW (Research and Analysis Wing)"
The Pain of Losing a Nation. Story of Lhendup Dorji and Sikkim | United We Blog! for a Democratic Nepal
India’s “Chief Executive” in Gangtok wrote: “Sikkim’s merger was necessary for Indian national interest. And we worked to that end. Maybe if the Chogyal had been smarter, and played his cards better, it wouldn’t have turned out the way it did.”
India’s illegal occupation of independent Sikkim has to be reversed | Proud to be a Sikkimese