What was the cause of the Sino-Indian War? Which area was disputed?

It was because the two sides disagreed on the demarcation of the border. India insisted that the McMahon Line in Tibet was the demarcation line between China and India, which the Chinese side did not accept. This involves about 90,000 square kilometers of land south of the Himalayas.

The two sides went to war over this in 1962, with our government claiming that we won the battle, but still withdrew north of the McMahon Line. India then occupied the disputed land, which is now home to nearly 10 million Indians.

It is worth noting that that land has a subtropical climate, quite different from Tibet proper, and is suitable for a large population. China actually has a lot to lose by giving up this disputed land. And we have to deduct at least these 90,000 square kilometers from our nominal 9.6 million square kilometers of land.