India last week turned nearly two decades of Pakistan policy on its head. By announcing attacks on terrorist “launch pads”—final staging posts for militants before they cross over from the Pakistan-controlled part of Kashmir to the Indian side—New Delhi signaled a new pugnaciousness in response to terrorism from across the border, such as the Sept. 18 attack that killed 19 Indian soldiers at a Kashmir border camp.

In another departure from the past, the U.S. appeared to back India rather than reflexively call for restraint...