A young English clerk in London drew a few lines on a map, this is China, this is the British Empire. A few other clerks took out their pencils and rulers, and drew on the maps of Africa and the Middle East to define their national borders. This was the days of the British ruling the world. They cut up countries to their likings, with no respect to the rights of any country. They were the Empire that ruled the world, set the rules, who owns what and where. The colonies had no say. The British decided what is right and wrong.
When McMahon drew the lines, cutting South Tibet from China as part of the British Empire, China protested, China never agreed to it till today. It was Chinese territory, not for the British Empire to decide, not for a young clerk in London to decide. Take this case to any international court, to the UN for a legal interpretation. No reasonable man or reasonable judge today or even in the days of the British Empire, would rule that this is legitimate.
When the British gave up their Empire, letting colonies to go independent, the newly independent India conveniently wanted to take over whatever that belonged to the British Empire, including South Tibet that belonged to China but seized by the Brits. India is not the British Empire. Whatever land the Brits stolen from their colonies were and should rightfully returned to their rightful owners. Lands in Africa, the Middle East, SE Asia and the rest of the world were returned, and many with enforced boundaries dictated by the Brits but not acceptable to their original owners.
India got their lands back, but wanted more, including lands the Brits took from China. The Indians audaciously claimed the Chinese land as theirs, using the lines draw by the British clerk McMahon. Is this reasonable, legally acceptable, legitimate? The Indians did not care. It is like money in their pockets, regardless of how it got in there, once inside India's pocket, do not expect the Indians to cough it out. When they realised that their argument basing on McMahon lines were unacceptable, unreasonable, they turned around and said, China has so much land, why can't China let the Indians have the land? This is the Indian psyche. 野蛮心态.
Actually, China wanted to give the land to India, but on China's term. It must first be returned and then be given by China voluntarily. China would not allow the land to be seized by any country and taken from them against their will. But the Indians refused to accept the offer by then PM Zhou Enlai.
The border dispute will not be settled as long as India sticks to its ugly and unreasonable position. China was even willing to compromise, to negotiate, maybe let India take a certain part of the land. But arrogant India wants it all, not negotiable. Eventually the land would be settled by force, by war, if no wise Indian leader is prepared to face the historical truth. Chinese land stolen by the British does not automatically become Indian land. Period.
India can continue to be stubborn and unreasonable. Time is not on India's side. The world is watching how the Indian mind is at work. Just like all the foreign investments in India. Once money is invested in India, in Indian assets, they belong to India. Do not expect to get them back. The foreign investors would be the legal owners, India would be the de facto owner if not even the real owner given time. Only suckers would keep pouring money into India and think it is safe. So many foreign investors have been cheated by the Indians, robbed by the Indians. Some robbed of everything, including their pants, but pretending all is fine. As long as they are happy to leave their money and assets in India and let the Indians do as they like, everything superficially is fine. Just see what will happen when they want their money and assets back. Then they will know who is the real owner. Money inside India's pocket will become India's money even if they don't belong to India.