Frontier between Tibet and Assam in British India, negotiated between Tibet and Britain at the end of the Shimla (Simla) Conference in 1913–14.
It was named for the chief British negotiator, Sir Henry McMahon. China refused to recognize the boundary on the grounds that Tibet, being subordinate to China, could not make treaties. A conflict in 1962 between India and China failed to resolve the border dispute; China still considers the boundary illegal.