By reaching a deal with India on civil nuclear co-operation, the US has overcome the single greatest obstacle to a long-awaited rapprochement between the world's two biggest democracies: New Delhi's resentment at its status as a nuclear pariah.

Advocates of a new relationship between the two "natural allies" see the potential for a strategic partnership capable of redefining the geopolitics of the 21st century. The US, which only recently sought to balance India and Pakistan and to deny the larger country pre-eminence in south Asia, now openly supports its emergence as a great power.