I have watched a documentary produced by Phoenix Satellite Television based on HongKong. It was mentioned in the documentary that before PLA marched into Tibet, Nehru was planning to forestall PLA by sending Indian troop to invade Tibet. However, India was having a conflict with Pakistan at that time, and he didn't have any spare troop that could be dispatched to Tibet. Therefore, Nehru perished that thought.
As far as i know, India has been harbouring a territorial ambition over Tibet for a very long time, which dates back to the time before the Independence of India. Nehru had a very ambitious plan of building a country made of all the countries in today's South Asia. And that plan includes separating Tibet from China, turning it into a buffer region between China and India, and annexing it when necessary.
India's ambition over Tibet explains well why she accepted Dalai Lama in the first place. And by refusing to stop hosting Dalai Lama and supporting those Tibetan secessionists, India is clearly sending us a message that her ambition over Tibet is not over yet.