China occupied Tibet militarily, whereas it was the Legislature of Sikkim which (under the leadership of Mr. Bhandari) opted to join the Indian territory. What made the said Legislature to act the way it did, is not public knowledge--though a few Indian & Sikkimese officials/people do know. Moreso, it happened during the time of a very strong Prime Minister, viz. Mrs Indra Gandhi. I know because i was in the Armed Forces and my Brother was in the Intellegence at that time.