It literally was out of the blue. There were minimal troops stationed there. India had streets named after the friendship agreement with China, there were slogans that went “Indians and Chinese are brothers”. Brothers can have minor disagreements, but we have their back and they have hours.

The border was guarded about as well as the US-Canada border on the Indian side.

The surprise war caught India completely off guard. The surprise was so effective that China captured all the territory they wanted, killed thousands of Indian troops and declared a unilateral ceasefire before India could marshal a response.

Prime Minister Nehru was badly shocked by this betrayal. He never recovered from it. His health deteriorated rapidly and he died a little over a year later.

Consider that maybe a 2 minute skim of Wikipedia teaches you very little. Certainly not as much as reading many books on this subject, which I have.