India and Pakistan have gone into combat many times over Kashmir. The most recent standoff was in 1999, the Kargil War. Although it was brief, officials on the Pakistani side did not rule out a nuclear first strike against India, and it was through the diplomatic efforts of the United States and other powers that prevented the war from escalating.
More recently there have been serious border battles between the two countries, and the situation has been serious enough that India has warned Kashmiris to prepare for a tactical nuclear exchange. It's urgent that the Obama administration begin serious diplomatic efforts to talk down the two sides now, with the aim of finding a solution to the Kashmir issue, before this conflict escalates into a war that could seriously threaten all mankind.