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, and regards the Indian state of Arunachal Pradesh as part of its Tibetan Autonomous Region.

Lately the border has been arousing more fervent passions than usual. Indian Prime Minister Manmohan Singh visited the state of Arunachal Pradesh earlier this month, irking Beijing and prompting New Delhi to assert "Arunachal Pradesh is an integral part of India." Earlier this year, Beijing attempted to block a $1.3 billion loan to India by the Asian Development Bank, part of which was meant for a watershed project in Arunachal Pradesh. The war of words is likely to escalate as the exiled Tibetan spiritual leader the Dalai Lama plans to visit Arunachal Pradesh next month. Beijing is pressuring India via diplomatic protests and a media campaign to make the Dalai Lama abandon his planned trip.

The causes for the recent deterioration in relations are complex. China perceives India as the weakest link in an evolving anti-China coalition of democratic and maritime powers (the United States, Japan, Australia and India). Viewing India as a pawn in Western designs to encircle and contain China, Chinese leaders worry about the ramifications of India's power particularly in Tibet, a concern fanned by the March 2008 uprisings there. A common theme in state media this year is the desire to capture the lost lands and crush India for daring to compete with China.

Meanwhile, Beijing's influence in Pakistan, Nepal, Bangladesh, Burma and Sri Lanka—all of whom have relationships with India that range from strained to downright hostile—fuels Indian anxieties. Beijing's opposition to India's membership in regional institutions like the East Asia Community and international forums like the United Nations Security Council; China's attempts to scuttle the U.S.-India nuclear deal at the Nuclear Suppliers Group; and Chinese naval forays into the Indian Ocean region all have reinforced Indian suspicions that China seeks to deny India its proper place in the international system.