China launches an attack up the Burma road attacking both Bengals, trying to invade India?
This would require strange politics on the Chinese side, almost certainly requiring Mao's early death, and the faction that ends up on top has to 'prove' itself. Either a 'short victorious war' to rally people behind them, or if a general ends up on top, then it's to show their devotion to spreading the Revolution.
Lin Biao's coup could be such a catalyst, if it was real And happened.