The Lashkar-e-Taiba has blamed India for the Karachi airport attack, the latest of many signs that it may be preparing the ground for terror strikes. Mr. Modi promised to hit back, but can he?

Few Indians would have been up before dawn that morning after 26/11, as Prime Minister Manmohan Singh sat around his office, watching the images of Mumbai burning. He may have wondered if the country would be at war before it woke. Intelligence Bureau officers who had been listening-in to the attackers’ conversations with their commanders in Karachi, had told the Prime Minister that there was little doubt of Pakistani involvement. Dr. Singh seemed stirred: “the people of India will not forgive us if we do nothing,” an aide recalls him saying.

He chose, however, to ignore his instincts. In a speech delivered on November 27, even as the bodies of victims were still strewn on the ground, the Prime Minister promised upgraded security forces, and aggressive diplomacy — everything, other than punitive action against the perpetrators.

“Listen up,” wrote Haruki Murakami, “there’s no war to end all wars.” Dr. Singh’s generals told him much the same thing, and he heard them.

“They did nothing,” said the man who has become India’s next Prime Minister, in a campaign speech centred on 26/11, “Indians died and they did — nothing.” “Talk to Pakistan in Pakistan’s language,” he went on, “because it won’t learn lessons until then.”

Ever since that morning, India’s strategic community has discussed what the country ought to do when the next 26/11 happens. Prime Minister Narendra Modi, India’s intelligence services fear, may have to answer that question sooner than most people expect. The Lashkar-e-Taiba (LeT) staged its first operation against an Indian target since 26/11 just hours before Mr. Modi took office, attacking India’s mission in Herat. Lashkar chief Hafiz Muhammad Saeed blamed Mr. Modi for this week’s attacks on Karachi — and demanded vengeance.

From past evidence, we know these threats aren’t idle. “The only language India understands is that of force,” a press release issued by the Lashkar’s parent organisation, Jamaat-ud-Dawa recorded Mr. Saeed as saying before 26/11, “and that is the language it must be talked to in.”

From files he will have read since taking office, Mr. Modi will have learned why Dr. Singh did nothing. Indian combat jets could hit training camps across the Line of Control (LoC), Air Chief Marshal Fali Major said at a November 29 meeting called for by the Prime Minister, but precise coordinates and adequate imaging weren’t available. Later, General Deepak Kapoor, the chief of Army staff, told Dr. Singh he couldn’t promise special forces’ strikes would be successful either.

No one could guarantee missile strikes wouldn’t escalate into war, or even a nuclear exchange — and no one could guarantee war would compel Pakistan’s military to change course.

Mr. Modi’s advisers know five responses are on the table — but all of them involve great risks. The first is to keep doing nothing. The threat of a major India-Pakistan crisis after 26/11 led the United States to mount intense pressure on Pakistan. In the years since, the LeT hasn’t mounted a single major operation on Indian soil. Its affiliate, the Indian Mujahideen, has had restraint thrust upon it by Pakistan’s Inter-Services Intelligence (ISI) Directorate.

Inaction, though, only succeeds if someone else does the hard work. In 2008, fearing its own war in Afghanistan would be undermined by an India-Pakistan war, the U.S. stepped in. Now, as it prepares to withdraw from Afghanistan, the country’s appetite for playing global policeman is diminishing. Doing nothing could thus invite even more punishment.

Like Prime Minister Atal Bihari Vajpayee, Mr. Modi’s second choice might be coercion. In 2001, after terrorists attacked Parliament, India mobilised troops. Pakistan was forced to respond — but its smaller economy suffered disproportionately. The stratagem is time-tested. In 1953, Prime Minister Jawaharlal Nehru mobilised troops in Punjab to deter a Pakistani attack into Kashmir.

Mr. Vajpayee’s strategy worked, forcing Pakistan to dramatically scale down the jihad in Jammu and Kashmir. It only worked, though, because the U.S. played mediator — much like after 26/11 — and it was hideously expensive, in money and lives.

The third choice is to do what Dr. Singh couldn’t: limited strikes on jihadist training camps across the LoC, using air power or missiles. In the five years since 26/11, India’s ability to conduct such strikes has been significantly enhanced. However, the tactic hasn’t had great success. In August 1998, the United States fired missiles into Afghanistan, seeking to avenge bombings which killed 224 people. In all, 75 missiles, each priced at $1.5 million, killed six minor jihadists. Moreover, Pakistan could hit back, targeting Indian industrial infrastructure, which is much more expensive than tent-and-donkey cart training camps.

Fourth, the Prime Minister could tell Indian troops to target the Pakistan Army along the LoC, using artillery and infantry — a task aided by the fact that its defences along stretches of the Neelam valley have been degraded by troops having to be moved to fight the Tehreek-e-Taliban elsewhere. The fighting that will follow though will make it more difficult to secure the LoC against jihadist infiltration — leading to heightened violence in Kashmir.

Mr. Modi could, finally, authorise the use of covert means, like bomb-for-bomb strikes or targeted assassination of jihadist leaders. Mr. Modi’s intelligence services, though, don’t have this arrow in their quiver — and it will be a while, most experts say, before they can acquire it.

Every sane person should hope Mr. Modi will never be required to exercise any of his military options — but thinking through war is just as important as talking peace.

Less than a week after the 26/11 strikes, Lieutenant General Shuja Pasha, the ISI chief, briefed a small Pakistani group off the record. He described Tehreek-e-Taliban commander Mullah Fazlullah — the architect of the Karachi strike — as a “true patriot.” Earlier, in April 2004, Taliban commander Nek Muhammad Wazir stood next to XI corps commander Syed Safdar Husain, promising that, in a war with India, he would be “Pakistan’s atomic bomb.” The generals weren’t crazy; for Pakistan’s Army, mired in a losing war against the jihadists it once nurtured, hostilities with India offer the sole hope of repairing its relationship with the jihadists.

Pakistani military commanders know their on-off war against jihadists has no hope of glorious victory: their resources are too thin, and many in their own ranks are sympathetic to the enemy. India is the enemy they need to restore their legitimacy as the Praetorian Guard of the national project. In the face of threats from an existential adversary, their enemies would be compelled to fall in line.

It is no coincidence that the Lashkar was authorised to carry out 26/11 just as Gen. Pervez Kayani became army chief, and set about undoing the damage his predecessor had inflicted by taking on the jihadists. Gen. Raheel Sharif, his reluctant offensive in North Waziristan already flailing, will find a crisis to the east to be a relief.

Pakistan Prime Minister Nawaz Sharif may wish to build a durable relationship with India, but he is no position to defy the generals. He is beholden, moreover, to Islamists who aided his election.

Less than six weeks before 26/11, it is mostly forgotten, Pakistan President Asif Ali Zardari imagined “Pakistani cement factories being constructed to provide for India’s huge infrastructure needs, Pakistani textile mills meeting Indian demand for blue jeans, Pakistani ports being used to relieve the congestion at Indian ones.” Even as Mr. Vajpayee and Mr. Sharif signed the Lahore Declaration in 1999, we now know, Pakistani troops were being trained to push their way across the LoC.

The truth, as Pakistani defence analyst Ayesha Siddiqa has noted, is that Pakistan’s “democratic transition does not mean the army is ready to surrender its control over security and foreign policies.”

New Delhi has long hoped that engaging Pakistan’s democratic leadership will catalyse that transformation. It has absorbed the blows dished out by the Army, hoping things will eventually change. Doing nothing, though, has proved both politically unsustainable and strategically ineffectual.

From Jawaharlal Nehru on, every Indian Prime Minister has faced this impossible challenge: how to punish the Pakistan Army’s sponsorship of terrorism, but ensure victory doesn’t come at a price the country cannot afford. Prime Minister Indira Gandhi, who ripped Pakistan in two, lost her life to the ISI-backed insurgency in Punjab.

Now that the roseate glow of his inaugural has subsided, Mr. Modi needs to listen to his advisers, and think through the prospects — and costs — of war. The future of a subcontinent will depend on the choices he makes.

praveen.swami@thehindu.co.in