Very interesting tidbit regarding the historical circumstances of this dispute, over at China Matters.

The Simla Agreement was apparently treated as an aspirational document and was recorded in the most authoritative compendium of British Indian treaties, Sir Charles Umpherston Aitchison’s Collection of Treaties, Engagements, and Sanads, with the notation that neither Great Britain nor China had ratified the treaty.  China, indeed, never accepted the McMahon Line.  Since Tibet wasn’t recognized as a sovereign power, whatever it hoped to achieve with the Simla Accord—and what it had tried to give away, namely Tawang– was, in the eyes of the British, moot.

Things puttered along until 1935, when the detention of a British spy in Tawang by Tibetan authorities awakened the cupidity of a diplomat in the Foreign Office of British India, Olaf Caroe.
Caroe checked the files, found that Great Britain had no ratified claims on Tawang, and decided to amend and improve the record.

He arranged for the relevant original volume of the 1929 Aitchison compendium to be withdrawn from the various libraries in which it was filed, discarded, and replaced with a new version—but one that still claimed to be compiled in 1929, thereby removing the need for awkward explanations or documentation concerning why the switch had happened. The spurious version claimed that Tibet and Britain had accepted the treaty.  Thereby, the unsurveyed McMahon Line was repurposed as a sacrosanct British imperial border, and Tawang was slotted into the British Indian side of the ledger.

The deception was only discovered in 1964, when a researcher was able to compare one of the last three surviving copies of the original compendium, at Harvard University, with the spurious replacement.

Unfortunately, that was too late for Nehru, who staked his security strategy and his diplomatic exchanges with China to a significant extent on the fallacy that he had inherited from British India a clear and unequivocal claim to its borders.

In 1962 Nehru decided to move up military units to assert India’s claim to contested territory in Ladakh/Aksai Chin and in Arunachal Pradesh under a gambit optimistically named The Forward Policy. Unluckily for Nehru, Chairman Mao was itching to stick it to India’s patron, Nikita Khrushchev, and the PLA attacked with overwhelming force on both fronts.   India’s entire strategy had been predicated on the assumption that the PRC would not respond (shades, I think, of Western confidence that Vladimir Putin would stay his hand in eastern Ukraine out of fear of sanctions and the wrath of his impoverished and disgruntled oligarchs) and the Indian Army, outnumbered, undersupplied, and disorganized, was completely unprepared for a desperate fight on the remote, high altitude battlefields.

India suffered a humiliating defeat at the hands of the PLA. After its victory, the PRC decided to take the high ground, diplomatically as well as geographically. It withdrew its forces to behind the McMahon Line and offered negotiations of the boundaries based on the status quo, in other words a de facto swap of AP for AC.

No dice, as we have seen. India clearly does not see any need to credit Arunachal Pradesh—territory that the PRC abandoned—as any kind of bargaining chip concerning Aksai Chin. This is, perhaps, a cautionary tale to the PRC as to the geostrategic minuses as well as pluses of trying to behave like Mr. Nice Guy.

This history is officially persona non grata in India. The report the Indian government commissioned on the 1962 war—the Henderson Brooks Report–was so devastating to India’s position and its legal, military, and diplomatic pretensions it was promptly banned and publication is forbidden to this day. In an ironic recapitulation of the case of the Aitchison compendium, it was assumed that there were only two typewritten copies and they were securely buttoned up in safes in New Delhi. However, the Times of London correspondent, Neville Maxwell, promptly got his hands on a copy and used it to write an expose on the tragedy of errors in 1962, India’s China War, thereby earning himself the fierce hatred of generations of Indian nationalists.

Maxwell tried several times to put the report into the public domain.

As quoted in Outlook India, Maxwell provided an interesting account of how the freedom of expression sausage gets made when the information involved is not necessarily a matter of national security (the report is classified Top Secret, but its content—the minutiae of military decisions and movements fifty years ago–has no current strategic or tactical significance) but is a matter of supreme political embarrassment (to Nehru, the Congress Party, the Gandhi political dynasty, and to the army).

My first attempt to put the Report itself on the public record was indirect and low-key: after I retired from the University I donated my copy to Oxford’s Bodleian Library, where, I thought, it could be studied in a setting of scholarly calm. The Library initially welcomed it as a valuable contribution in that “grey area” between actions and printed books, in which I had given them material previously. But after some months the librarian to whom I had entrusted it warned me that, under a new regulation, before the Report was put on to the shelves and opened to the public it would have to be cleared by the British government with the government which might be adversely interested! Shocked by that admission of a secret process of censorship to which the Bodleian had supinely acceded I protested to the head Librarian, then an American, but received no response. Fortunately I was able to retrieve my donation before the Indian High Commission in London was alerted in the Bodleian’s procedures and was perhaps given the Report.

In 2002, noting that all attempts in India to make the government release the Report had failed, I decided on a more direct approach and made the text available to the editors of three of India’s leading publications, asking that they observe the usual journalistic practice of keeping their source to themselves. … To my surprise the editors concerned decided, unanimously, not to publish… Later I gave the text to a fourth editor and offered it to a fifth, with the same nil result.