As the world’s largest democracy and an economy that is of increasing importance to the United States, India’s power is central to the calculus of stability in the region.
An Indian deployment in Afghanistan projects a two-front threat to Pakistan... the eastern anvil of Kashmir and India itself and a western hammer of Indian forces in Afghanistan that can project Indian power against Pakistan in an easterly direction.
India has supported President Karzai in the past, and will bear the brunt of a resurgent Islam if the Taliban triumphs in Afghanistan. Such result will lead to serious instability in the region particularly given Pakistan’s own instability in the face of Islamic fundamentalism.
Moreover, unlike the United States, the Pakistanis know that India will not leave the region. Consequently, for Pakistan, the die is cast: Either get your house in order or India with the support of the United States will do it for you.