One of the more tenacious conspiracy theories that have taken root in the hothouse of Pakistan's capital is that Osama bin Laden was not killed in the May 2, 2011, Navy SEAL raid on his compound in Abbottabad -- that, in fact, he had already been dead for years, killed in the caves of Tora Bora.

According to this theory, the CIA had been keeping bin Laden's corpse on ice, literally, ready to be resurrected at a moment when his "death" could better serve U.S. interests. That moment came when the SEALs decided to conduct a dry run of their long-planned operation to snatch Pakistan's nuclear weapons. Bin Laden's thawing corpse was brought along as cover in case the exercise blew up -- and as a devious bit of political theater to besmirch Pakistan's reputation if all went well.

What keep conspiracy theories like this alive are bits and pieces of half-baked evidence that could be construed to support a deeply held belief. In this case, it is the belief -- accepted across the board in Pakistan, from the top brass of its military down to the dusty gaggle of taxi drivers who awaited me each morning outside my Islamabad hotel -- that the United States has a not-so-secret plan to snatch Pakistan's nuclear arsenal.

The United States, which is duly concerned that Pakistan's nukes could fall into the wrong hands, almost certainly does have a plan to neutralize those weapons in the event of a coup or total state collapse. When the question was put to Condoleezza Rice during her 2005 confirmation hearings to become secretary of state, she replied, "We have noted this problem, and we are prepared to try to deal with it."

"Try" is the key word. Military experts -- American, Pakistani, and Indian -- agree that grabbing or disarming all of Pakistan's nukes at this stage would be something close to mission impossible. As one senior Pakistani general told me, "We look at the stories in the U.S. media about taking away our nuclear weapons and this definitely concerns us, so countermeasures have been developed accordingly." Such steps have included building more warheads and spreading them out over a larger number of heavily guarded locations. This, of course, also makes the logistics of securing them against theft by homegrown terrorists that much more complicated.

Fears of that terrifying possibility were heightened in August, when a group of militants assaulted a Pakistani base that some believe houses nuclear weapons components. Nine militants and one soldier were killed in a two-hour firefight at the Kamra air force base. The local media immediately floated the theory that this, too, was part of the American plot to steal Pakistan's nukes. But more disturbing than any conspiracy theory is the reality that this was the fourth attack in five years on the Kamra base, just 20 miles from the capital. At least five other sensitive military installations have also come under attack by militants since 2007.

Yet, though the danger of a loose Pakistani nuke certainly deserves scrupulous attention, it may not be the severest nuclear threat emanating from South Asia, as I came to realize after interviewing more than a dozen experts in Pakistan, India, and the United States this summer. Since the 9/11 attacks, preventing the world's most dangerous weapons from falling into the hands of the world's most dangerous actors -- whether al Qaeda terrorists or Iranian mullahs -- has understandably been America's stated priority. Yet the gravest danger -- not only for the region, but for the United States itself -- may be the South Asian incarnation of a Cold War phenomenon: a nuclear arms race.

Pakistan, with an estimated 90 to 120 warheads, is now believed to be churning out more plutonium than any other country on the planet -- thanks to two Chinese-built reactors that are now online, a third that is undergoing trials, and a fourth that is scheduled to become operational by 2016. It has already passed India in total number of warheads and is on course to overtake Britain as the world's No. 5 nuclear power. Pakistan could end up in third place, behind Russia and the United States, within a decade.

This April, Pakistan tested a short-range ballistic missile, the Hatf IX, a so-called "shoot and scoot" battlefield nuclear weapon aimed at deterring an invasion by India's conventional forces. This development carries two disturbing implications. First, Pakistan now has the know-how to build nuclear warheads compact enough to fit on the tip of a small missile or inside a suitcase (handy for terrorists). Second, Pakistan has adopted a war-fighting doctrine that does not preclude nuking its own territory in the event of an Indian incursion -- a dubious first in the annals of deterrence theory.

India, meanwhile, has just tested its first long-range ballistic missile, the Agni-V, with a range of 3,100 miles. In April, the Indian Navy added a new Russian-made nuclear-powered submarine to its fleet and is now building its own nuclear subs. One has already been launched and will enter service next year, and India is determined to add submarine-launched ballistic missiles to its arsenal. This puts India on the verge of joining the elite nuclear "triad" club -- states with the ability to survive a first strike by an adversary and deliver a retaliatory strike by land, sea, or air.

India has also said that it has successfully tested an anti-ballistic missile shield that could be deployed "in a short time" to protect New Delhi and Mumbai. The downside of this defensive measure -- putting aside the question of effectiveness -- is that it invites an adversary to build many more warheads in the hope that a few will be able to slip through the shield.

India claims that it is not really engaged in an arms race -- or that, if it is, its opponent is not Pakistan, but China, a nuclear-armed superpower and economic rival with which it shares a disputed border. The Agni-V was dubbed the "China-killer" in some overheated Indian headlines. China's nuclear ambitions are geared toward deterring the United States and Russia, but it obligingly stirs the pot in South Asia by providing Pakistan with plutonium reactors -- in flagrant violation of its obligations as a member of the Nuclear Suppliers Group.

Meanwhile, through a 2008 deal negotiated by George W. Bush's administration, the United States has given India access to nuclear fuel on the international market. In the past, India had been barred from such trade because the Nuclear Non-Proliferation Treaty does not consider its nuclear weapons program legitimate, and its limited supplies of domestic uranium forced it to choose between powering its reactors and building more nuclear weapons. "Power production was the priority; now they can have both," explained Toby Dalton, deputy director of the Nuclear Policy Program at the Carnegie Endowment for International Peace.

With both sides armed to the teeth, it is easy to exaggerate the fears and much harder to pinpoint where the real dangers lie. For the United States, the nightmare scenario is that some of Pakistan's warheads or its fissile material falls into the hands of the Taliban or al Qaeda -- or, worse, that the whole country falls into the hands of the Taliban. For example, Rolf Mowatt-Larssen, a former CIA officer now at Harvard University's Belfer Center for Science and International Affairs, has warned of the "lethal proximity between terrorists, extremists, and nuclear weapons insiders" in Pakistan. This is a reality, but on the whole, Pakistan's nuclear arsenal appears to be reasonably secure against internal threats, according to those who know the country best.

To outsiders, Pakistan appears to be permanently teetering on the brink of collapse. The fact that large swaths of the country are literally beyond the control of the central government is not reassuring. But a weak state does not mean a weak society, and powerful internal dynamics based largely on kinship and tribe make it highly unlikely that Pakistan would ever fall under the control of an outfit like the Taliban. During the country's intermittent bouts of democracy, its civilian leaders have been consistently incompetent and corrupt, but even in the worst of times, the military has maintained a high standard of professionalism. And there is nothing that matters more to the Pakistani military than keeping the nuclear arsenal -- its crown jewels -- out of the hands of India, the United States, and homegrown extremists.

"Pakistan struggled to acquire these weapons against the wishes of the world. Our nuclear capability comes as a result of great sacrifice. It is our most precious and powerful weapon -- for our defense, our security, and our political prestige," Talat Masood, a retired Pakistani lieutenant general, told me. "We keep them safe."

Pakistan's nuclear security is in the responsibility of the Strategic Plans Division, which appears to function pretty much as a separate branch of the military. It has its own training facility and an elaborate set of controls and screening procedures to keep track of all warheads and fissile material and to monitor any blips in the behavior patterns of its personnel. The 15 or so sites where weapons are stored are the mostly heavily guarded in the country. Even if some group managed to steal or commandeer a weapon, it is highly unlikely the group would be able to use it. The greater danger is the theft of fissile material, which could be used to make a crude bomb. "With 70 to 80 kilos of highly enriched uranium, it would be fairly easy to make one in the basement of a building in the city of your choice," said Pervez Hoodbhoy, a distinguished nuclear physicist at Islamabad's Quaid-i-Azam University. At the moment, Pakistan has a stockpile of about 2.75 tons -- or some 30 bombs' worth -- of highly enriched uranium. It does not tell Americans where it is stored.

"All nuclear countries are conscious of the risks, nuclear weapons states especially so," said Gen. Ehsan ul-Haq, who speaks with the been-there-done-that authority of a man who has served as both chairman of Pakistan's Joint Chiefs of Staff Committee and head of the ISI, its controversial spy agency. "Of course there are concerns. Some are genuine, but much of what you read in the U.S. media is irrational and reflective of paranoia. Rising radicalism in Pakistan? Yes, this is true, and the military is very conscious of this."

Perhaps the most credible endorsement of Pakistan's nuclear security regime comes from its most steadfast enemy. The consensus among India's top generals and defense experts is that Pakistan's nukes are pretty secure. "No one can be 100 percent secure, but I think they are more than 99 percent secure," said Shashindra Tyagi, a former chief of staff of the Indian Air Force. "They keep a very close watch on personnel. All of the steps that could be taken have been taken. This business of the Taliban taking over -- it can't be ruled out, but I think it's unlikely. The Pakistani military understands the threats they face better than anyone, and they are smart enough to take care it."

Yogesh Joshi, an analyst at the Institute for Defense Studies and Analyses in New Delhi, agrees: "Different states have different perceptions of risk. The U.S. has contingency plans [to secure Pakistan's nukes] because its nightmare scenario is that Pakistan's weapons fall into terrorist hands. The view from India over the years is that Pakistan, probably more than any other nuclear weapons state, has taken measures to secure its weapons. At the political level here, there's a lot of confidence that Pakistan's nuclear weapons are secure."

The greater concern -- not only for India and Pakistan, but for the United States and everyone else -- may be the direct competition between the two South Asian states. True, in terms of numbers and destructive capacity, the arms buildup in South Asia does not come close to what was going on during the Cold War, when the United States and the Soviet Union built enough bombs to destroy the planet many times over. India and Pakistan have enough to destroy it only once, perhaps twice.

But in many ways, the arms race in South Asia is more dangerous. The United States and the Soviet Union were rival superpowers jockeying for influence and advantage on the global stage, but these were also two countries that had never gone to war with each other, that had a vast physical and psychological separation between them, that generally steered clear of direct provocations, and that eventually had mechanisms in place (like the famous hotline between Moscow and Washington) to make sure little misunderstandings didn't grow into monstrous miscalculations.

By contrast, the India-Pakistan rivalry comes with all the venom and vindictiveness of a messy divorce, which, of course, it is. The two countries have officially fought three wars against each other since their breakup in 1947 and have had numerous skirmishes and close calls since then. They have a festering territorial dispute in Kashmir. The 1999 Kargil conflict, waged a year after both countries went overtly nuclear, may have come closer to the nuclear brink than even the 1962 Cuban missile crisis. At the height of the showdown, there was credible intelligence that both sides were readying their nuclear arsenals for deployment.

Pakistan lost all three of these wars. Its very large army is still only half the size of India's, whose military budget is more than seven times larger than Pakistan's. Pakistan's generals are well aware that in any all-out conventional confrontation with India, they're toast. The guiding ideology of Pakistan's Army -- from the generals on down to their drivers -- is that India represents a permanent existential threat. This is why Pakistan clings to its nukes and attempts to maintain at least the illusion of what its generals call "bilateral balance."

This conventional asymmetry increases the danger of the nuclear arms race -- it feeds India's hubris and Pakistan's sense of failure. Here are two countries headed in opposite directions. India's $1.7 trillion economy is eight times the size of Pakistan's and has grown at an enviable 8.2 percent annually over the last three years, compared to just 3.3 percent for Pakistan. India is in the forefront of the digital revolution, and while the country's leaders were embarrassed by this summer's massive two-day blackout, Pakistan's broken-down infrastructure struggles to provide citizens with more than a few hours of electricity each day. India, the world's largest democracy, is on the cusp of becoming a global power; Pakistan, with its on-and-off military dictatorships (off at the moment), ranks 13th on Foreign Policy's most recent Failed States Index.

More significant than these statistics is the mindset behind them. India is brimming with confidence. Pakistan is hobbled by fear, paranoia, and a deep sense of inferiority. India's major cities, New Delhi and Mumbai, are modernizing global metropolises. Checking into the Marriott in Pakistan's capital is like checking into a maximum-security prison -- high walls topped with razor wire, armed guards in watchtowers. Islamabad today looks and feels like a city under siege where there could be a coup at any moment. Soldiers and checkpoints are everywhere. It felt this way the first time I visited, in 1985.

This economic and cultural lopsidedness is strikingly reflected in the countries' nuclear competition.

In perhaps no other major power is the military quite so submissive to civilian authority as it is in India. "The civilian side lords it over the military in a manner that often borders on humiliation -- and there is no pushback from the military," said Ashley Tellis, an India expert with the Carnegie Endowment. The reasons for this are rooted in India's long struggle for independence against a colonial master that filled the ranks of its police and army with natives. "The military was seen as a force that served a colonial occupier," said Tellis. With the Indian officer corps' fondness for whiskey, mustaches, and other Briticisms, "the nationalist leadership looked at them as aliens" and took extreme measures to make sure there would be no coups.

From a nuclear standpoint, the result of this dynamic is a command-and-control system that is firmly in the hands of the civilian political leadership, a clearly stated "no first use" policy, and a view that nukes are political weapons -- a way to project global power and prestige -- not viable war-fighting tools.

In theory, Pakistan's nuclear trigger is also in civilian hands. A body called the National Command Authority, headed by the prime minister, is supposed to be the ultimate decider of whether to initiate a nuclear attack. In reality, however, it is the military that controls the process from top to bottom. Pakistan has never formally stated its nuclear doctrine, preferring to keep the Indians guessing as to when and where it might use nukes. But now it appears to be contemplating the idea of actually using tactical nuclear weapons in a confrontation with India.

The problem with this delicate state of affairs is not simply the two countries' history of war, but Pakistan's tactic of hiding behind its nuclear shield while allowing terrorist groups to launch proxy attacks against India. The 2001 attack on India's Parliament building and the 2008 Mumbai attack are the most egregious examples. Both were carried out by Lashkar-e-Taiba militants based in Pakistan with well-established links to the ISI and were far more provocative than anything the Americans or Russians dished out to each other during the four decades of the Cold War. (More than 160 people were killed in the attack that held India's largest city hostage for 60 hours.) Terrorism is the classic underdog tactic, but Pakistan is certainly the world's first nuclear-armed underdog to successfully apply the tactic against a nuclear rival.

India has been struggling to respond. "For 15 years this country is bleeding from attack after attack, and there is nothing we can do," said Raja Mohan of the Observer Research Foundation, a New Delhi think tank. "The attacks correlate directly to Pakistan's acquisition of nuclear weapons. From the moment they got nukes, they saw it as an opportunity they could exploit. And India has no instruments to punish Pakistan or change its behavior."

There are encouraging signs that Pakistan may be rethinking this tactic, realizing that over the long run the Taliban and others of its ilk pose a far greater danger to Pakistan than to India. The relentless succession of suicide bombings and attacks on police and military bases and a costly war to wrest control of the Swat Valley from the Taliban seem to have finally convinced Pakistan's military that, in the words of one general, "the threat today is internal, and if it is not pushed back and neutralized, it will continue to expand its influence and we will have an Afghanistan situation inside our own country." But even if the ISI is sincere about ending its relationship with jihadi proxies, India's military planners are still searching for an appropriate weapon with which to punish Pakistan in the event of "another Mumbai."

The problem for India is that even though it holds a huge advantage in conventional forces, its mobilization process is ponderously slow. This shortcoming was humiliatingly exposed after the 2001 attack on the Parliament building, when it took the Indian Army about three weeks to deploy for a retaliatory strike -- enough time for the United States to step in and cool tempers on both sides. A potential nuclear crisis had been averted, but in 2004, India, still smarting from its inability to retaliate, announced a new war-fighting doctrine dubbed "Cold Start," which called for the capability to conduct a series of cross-border lightning strikes within 72 hours. The idea was not to hold territory or threaten the existence of the Pakistani state, but to use overwhelming firepower to deliver a punishing blow that would fall short of provoking a nuclear response.

Pakistan's reaction -- or overreaction -- was to double down on developing its short-range battlefield nuclear weapon, the Hatf IX. Any incursion from India would be met with a nuclear response even if it meant Pakistan had to nuke its own territory. "What one fears is that with the testing of these short-range nuclear missiles -- five in the last couple of months -- this seems to indicate a seriousness about using theater nuclear weapons," said Hoodbhoy, the physicist.

While strategists on both sides debate whether the Hatf IX, with a range of 60 kilometers and a mobile multibarrel launch system, would be enough to stop an advancing column of Indian tanks -- Hoodbhoy argues that "smaller, sub-kiloton-size weapons are not really effective militarily" -- they do agree that it would take more than one missile to do the job, instantly escalating the crisis beyond anyone's control.

The last nuclear weapon state to seriously consider the use of battlefield nuclear weapons was the United States during the first decades of the Cold War, when NATO was faced with the overwhelming superiority of Soviet conventional forces. But by the early 1970s, U.S. strategists no longer believed these weapons had any military utility, and by 1991 most had been withdrawn from European territory.

Pakistan, however, seems to have embraced this discarded strategy and is now, in effect, challenging India to a game of nuclear chicken -- which seems to have made India tread carefully. Tellingly, in 2008, when Lashkar terrorists attacked Mumbai, Cold Start was not implemented. These days, Indian officials seem to be backing away from the idea. "There is no Cold Start doctrine. No such thing. It was an off-the-cuff remark from a former chief of staff. I have been defense minister of the country. I should know," veteran Indian politician Jaswant Singh assured me. In a WikiLeaked classified document dated Feb. 16, 2010, Tim Roemer, then U.S. ambassador to India, described Cold Start as "a mixture of myth and reality" that, if implemented, "would likely encounter very mixed results."

Pakistani military planners, however, continue to be obsessed with the idea of Cold Start. It comes up in every conversation about security, and it is the driving force behind the country's program to develop tactical battlefield nukes. For now, the focus is on missile delivery systems, but according to Maria Sultan, director of the South Asian Strategic Stability Institute, an Islamabad think tank, there is growing interest in using nukes in other ways -- such as to create an electromagnetic pulse that would fry the enemy's electronics. "In short, we will look for full-spectrum response options," she said.

The arms race could make a loose nuke more likely. After all, Pakistan's assurances that its nuclear arsenal is safe and secure rest heavily on the argument that its warheads and their delivery systems have been uncoupled and stored separately in heavily guarded facilities. It would be very difficult for a group of mutinous officers to assemble the necessary protocols for a launch and well nigh impossible for a band of terrorists to do so. But that calculus changes with the deployment of mobile battlefield weapons. The weapons themselves, no longer stored in heavily guarded bunkers, would be far more exposed.

Nevertheless, military analysts from both countries still say that a nuclear exchange triggered by miscalculation, miscommunication, or panic is far more likely than terrorists stealing a weapon -- and, significantly, that the odds of such an exchange increase with the deployment of battlefield nukes. As these ready-to-use weapons are maneuvered closer to enemy lines, the chain of command and control would be stretched and more authority necessarily delegated to field officers. And, if they have weapons designed to repel a conventional attack, there is obviously a reasonable chance they will use them for that purpose. "It lowers the threshold," said Hoodbhoy. "The idea that tactical nukes could be used against Indian tanks on Pakistan's territory creates the kind of atmosphere that greatly shortens the distance to apocalypse."

Both sides speak of the possibility of a limited nuclear war. But even those who speak in these terms seem to understand that this is fantasy -- that once started, a nuclear exchange would be almost impossible to limit or contain. "The only move that you have control over is your first move; you have no control over the nth move in a nuclear exchange," said Carnegie's Tellis. The first launch would create hysteria; communication lines would break down, and events would rapidly cascade out of control. Some of the world's most densely populated cities could find themselves under nuclear attack, and an estimated 20 million people could die almost immediately.

What's more, the resulting firestorms would put 5 million to 7 million metric tons of smoke into the upper atmosphere, according to a new model developed by climate scientists at Rutgers University and the University of Colorado. Within weeks, skies around the world would be permanently overcast, and the condition vividly described by Carl Sagan as "nuclear winter" would be upon us. The darkness would likely last about a decade. The Earth's temperature would drop, agriculture around the globe would collapse, and a billion or more humans who already live on the margins of subsistence could starve.

This is the real nuclear threat that is festering in South Asia. It is a threat to all countries, including the United States, not just India and Pakistan. Both sides acknowledge it, but neither seems able to slow their dangerous race to annihilation.