What happens if tensions between India and Pakistan boil over?

India and Pakistan inched closer to overt conflict last week after an attack in the Indian-administered Kashmir region left dozens dead and set the two nuclear-armed neighboring nations on the latest collision course in their decades of acrimonious relations. In the days since gunmen opened fire on tourists in Kashmir's Anantnag district, India has threatened to withdraw from the treaty that provides water to the bulk of Pakistan, prompting that country to close its airspace to Indian flights.