Officially, at least, both India and Pakistan say that chances of their current tensions escalating into a possible nuclear war are "unthinkable," "unlikely," and in the words of Pakistani President Pervez Musharraf, "insanity."
Yet wars seldom follow a neat plan. Military analysts say there are several conventional-warfare scenarios that could lead to a South Asia nuclear war:
India initiates a naval blockade of Karachi, cutting Pakistan off from the rest of the world. Pakistan feels its very survival is at stake, and launches nuclear missiles at India to retaliate.
India launches airstrikes that destroy the Karakoram Highway, which links Pakistan to its close ally, China. Pakistan launches nuclear missiles to retaliate.
Pakistani militants attack an Indian Army installation in Kashmir during the visit of a US official, such as Richard Armitage or Donald Rumsfeld. India responds with overwhelming force, launching a massive air and land assault on the Pakistani city of Lahore. Pakistan launches nuclear weapons to retaliate.
"It is very easy to envision scenarios under which this conflict does go nuclear, but they begin at the same premise: that there is a major ground war, and Pakistan is losing," says William Lind, a military analyst at the conservative Free Congress Foundation in Washington. The big question, and the essential "firebreak," he says, is not whether either country uses nuclear weapons or not, but the possibility of "using nuclear weapons symbolically versus massive use to flatten cities."
Pakistan could decide to use nuclear weapons against symbolic targets such as military installations or landmarks inside India if it is pushed up against a wall, he says. Or Pakistan could detonate a bomb over the ocean or the Indian desert to signal that it could do more.
Symbolic use would not entail the millions of casualties estimated, and would fall far short of even the US attack on Hiroshima and Nagasaki, but would still gain international reprobation nonetheless.
In these uncertain times, the one certain fact is that Indian military capability dwarfs that of Pakistan.
Up to 700,000 Indian Army and paramilitary forces are lined up along the Indo-Pakistani border and along the 450- mile cease-fire line in the northern state of Jammu and Kashmir, which both India and Pakistan claim. To meet this challenge, Pakistan has sent perhaps 300,000 of its own troops, and has repositioned troops from anti-Al Qaeda operations along the western border with Afghanistan to help repel any Indian invasion in Kashmir.
India's nuclear capability is also thought to be superior.
According to the Bulletin of the Atomic Scientists, Pakistan has from 12 to 18 nuclear warheads, each with 20 kilotons of power, similar in strength to the bomb the US dropped on Hiroshima in 1945. (Other estimates put Pakistan's capability as high as 30 warheads.)
India, by contrast, has an estimated 70 to 120 nuclear warheads of 20 to 30 kilotons in strength. It also claims to have tested a much more powerful hydrogen bomb in recent years, but its own tests show that India has not managed to harness the full explosive yield of this bomb.
To deliver these weapons, both Pakistan and India have tested ballistic missiles, and jet-aircraft delivery systems with tactical missiles that range up to 175 miles, and long-range missiles reaching up to 900 miles.
There is one further danger to consider.
Unlike the US and the former Soviet Union, India and Pakistan are neighbors, so there is precious little time five minutes to call back or shoot down any plane or missile carrying nuclear weapons.
"Neither side wants this to come to a nuclear war, but they have spent so much time discounting the chance of it happening that there is little preparation for the scenario where a mistake is made that triggers the other side, or moves in a conventional battle are misread," says David Albright, president of the Institute for Science and International Security in Washington, which promotes nonproliferation of nuclear weapons. "The problem is that in such scenarios there is very little time" to rectify missteps.
To be sure, there are plenty of efforts afoot to help India and Pakistan talk out rather than duke out their differences.
During a 16-nation Asian security summit that begins today in Almaty, Kazakhstan, Russian president Vladimir Putin is hoping to persuade both Pakistani President Musharraf and Indian Prime Minister Atal Behari Vajpayee to set out a road map toward de-escalating the conflict, and then, perhaps to work out a lasting solution. Later this week, Deputy Secretary of State Richard Armitage will visit Islamabad and New Delhi, followed by US Secretary of Defense Donald Rumsfeld.
Mr. Rumsfeld is expected to bring along a report from the US Defense Intelligence Agency, detailing estimated results of a nuclear exchange 12 million dead, drifting radiation, famine and to underline the point that the use of nuclear weapons would prompt the US to sever its relations with and curtail economic aid to either country.
Albright and other scientists say that, while a war employing 50 or more nuclear weapons would entail a large amount of fallout, it would deliver little or perhaps no deadly amounts to neighboring countries, because of the likely location of the bombings, the direction of winds, and the blocking effect of mountains.
As Vajpayee and Musharraf left for Almaty, their rhetoric took a more productive turn. Musharraf reiterated that he had ordered all militant infiltration into Kashmir to cease and was "ready to meet anywhere and at any level." Vajpayee responded, "if we see the result on the ground of Gen. Musharraf's [promises], we shall certainly give it our serious consideration."
In addition, most analysts in India say that the hyperbolic rhetoric by Indian and Pakistani leaders, most notably the Indian official who called on the Indian military to "level their cities," has more to do with placating their own restive domestic audiences rather than articulating a sincere thirst for nuclear war.
But even if the two rival leaders do begin a rapprochement, some analysts worry that uncontrolled militants may launch an attack on India, particularly when peace talks seem closest, regardless of what their former patrons in Islamabad say.
"Even if Musharraf is serious about controlling the militants, some of these groups especially Lashkar-e Tayyaba and Jaish-e Muhammad are totally financially independent," says Suba Chandran, a research associate at the Institute for Defense Studies and Analysis in New Delhi. "They have ways to infiltrate India and enough ways to wage war in Kashmir without Pakistan's help. This is the real threat."
But J.N. Dixit, India's former foreign secretary, says that if Musharraf is truly against terrorism and a genuine ally of the US, then his decision is rather simple.
"If he is really against terrorism and India acts to crush those terrorist camps in his territory, then Musharraf should join us," he says. "That is the logic of what he says."
But if Musharraf does launch a nuclear assault on India, Mr. Dixit says, "I think our second-strike capabilities are quite sufficient to neutralize Pakistan for 50 years at least."
If the "unthinkable" nuclear war does erupt, observers say it is most likely to start sometime after Rumsfeld's visit this Sunday but before the arrival of the monsoon season, which reaches northern India in the first week of July. Monsoon rains would make the logistics of fighting and supplying a conventional war in mountainous terrain all but impossible and thus greatly reduce the risk of escalation to a nuclear exchange.
The thing to watch in the next few days is body language and a change in rhetoric, says Edward Luttwak, an independent defense analyst based in Washington.
"Nobody launches a nuclear war willingly; they do it as a last resort because they are forced into it," says Mr. Luttwak.
"As long as these leaders are speaking in declarative sentences, such as 'We will level your cities,' then that shows these men still feel confident and in control, says Mr. Luttwak. "But if you see them start to speak in passive verbs, such as, 'It is out of our hands,' that is when it gets dangerous."