It has often been taken for granted that China and India will rise simultaneously and peacefully in the 21st century. But a recent flare-up challenges that view. Thirty-seven years after the two countries fought a border war and 28 years since they opened settlement negotiations, the entire frontier from Kashmir to Burma remains in question. It would be dangerous to ignore this festering sore any longer. The dispute stretches back to the British Raj, when colonial official Sir Henry McMahon drew the boundary between India and Tibet at the Shimla Convention in 1913. China has never recognized the McMahon Line,...