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In south Asia, there is no wall to tear down, as there was in Berlin. Yet the quarrel between two of the regions' great powers has lasted longer than the Cold War, and blown intermittently hot. Since independence from the British, India and Pakistan have persisted in sniping – and occasionally firing – at each other, each blaming the other, mutually suspicious, unable to reconcile despite their long shared history. Yet, as an American official pointed out this week, the two countries face a looming threat, one which seeks to pit them against each other – and one which they can only defeat together.

Speaking this week in New Delhi, Robert Gates, the US secretary of defence, warned that al Qa'eda and similarly inspired groups would seek to destabilise not only...