The armies of India and Pakistan often exchange fire across the front line in the disputed state of Kashmir. When tensions rise, one side will subject the other to a blistering artillery barrage. On occasion, the two have sent soldiers on forays into one another's territory.
But since the feuding neighbors tested nuclear weapons in the late 1990s, neither had dared send fighter jets across the frontier — until last week.
After a terrorist group based in Pakistan launched an attack in the Indian-controlled part of Kashmir that killed 40 soldiers, India responded by bombing what it said was a terrorist training camp in the Pakistani state of Khyber Pakhtunkhwa. Pakistan retaliated by sending jets to bomb Indian targets. In the ensuing air battle, both sides claim to have shot down the other's aircraft, and Pakistan captured an Indian pilot.
A miscalculation now could spell calamity. The fighting is already the fiercest between the two countries since India battled to expel Pakistani intruders from high in the Himalayas in 1999. The initial Indian air raid struck not Pakistan's bit of Kashmir, but well within Pakistan proper and just 100 kilometers from the capital, Islamabad. That, in effect, constituted a change in the rules of engagement between the two.
India and Pakistan are so often at odds that there is a tendency to shrug off their spats. But not since their most recent, full-blown war in 1971 has the risk of escalation been so high.
The intention of Narendra Modi, India's prime minister, in ordering the original airstrike was simple. Pakistan has long backed terrorists who mount grisly attacks in India, most notably in Mumbai in 2008, when jihadists who arrived by boat from Pakistan killed some 165 people. Although Pakistan's army promised then to shut down such extremist groups, it has not. By responding more forcefully than usual to the latest outrage, Modi understandably wanted to signal that he wasn't willing to allow Pakistan to keep sponsoring terrorism.
In the long run, stability depends on Pakistan ending its indefensible support for terrorism. Its prime minister, Imran Khan, is urging dialogue and, in a promising gesture, was due to release India's pilot — presumably with the approval of the army chief, who calls the shots on matters of security.
But in the short run, Modi shares the responsibility to stop a disastrous escalation. Because he faces an election in April, he faces the hardest and most consequential calculations. They could come to define his premiership.