Key Point: If peace ever collapsed, an Indian-Pakistani conflict might not stay regional. Fall out from any nuclear weapons use would devastate the entire world.

While the United States is preoccupied by the threat of nuclear weapons in the hands of potential adversaries such as Russia, China or North Korea, the danger of nuclear conflict may actually be greatest between two of its allies, Pakistan and India. The two nations have engaged in four wars starting since their partition along religious lines in 1947. A fifth could be drastically more costly, as their nuclear capabilities continue to grow and diversify.

Several years ago I made the acquaintance of a Pakistani nuclear science student in China. Curious about the thinking behind his country’s nuclear program, I asked if he really believed there was a possibility that India would invade Pakistan. “There’s still a lot of old-school thinkers in the Congress Party that believe India and Pakistan should be united,” he told me.

I doubt there are many observers outside of Pakistan who believe India is plotting to invade and occupy the Muslim state, but a feeling of existential enmity persists. The third conflict between the two countries in 1971 established India’s superiority in conventional warfare—not unexpectedly, as India has several times Pakistan’s population.

The bone of contention has always been the Indian state of Jammu and Kashmir. At the time of partition, the predominantly Muslim state was politically divided over which nation to join. When Pakistani-allied tribesmen attempted to force the issue, the Hindu maharaja of the region chose to accede to India, leading to the first war between India and Pakistan. Ever since, the line of control between the Indian and Pakistan side has remained bitterly contested, with artillery and sniper fire routinely exchanged. Pakistan intelligence services have infiltrated insurgents and plotted attacks across the border for decades, and Indian security troops have been implicated in human-rights violations and killings of the locals as a result of their counterinsurgency operations.

Pakistan does have to fear the potential of an Indian counterstrike intended to retaliate for a terrorist attack by Pakistani-aligned groups, such as the killing of 166 in Mumbai by Lashkar-e-Taiba in 2008 or the attack on Indian parliament in 2001 by Jaish-e-Muhammad. In both cases, the attackers had ties with Pakistani Inter-Services Intelligence, and Islamabad has shown limited willingness or ability to crack down on these groups. Complicating matters, civilian control of the military is far from consolidated in Pakistan, and it would be quite possible for ISI or some other agency to carry out such activities on its own initiative without the knowledge or support of the head of state.

India’s military has formulated a “Cold Start” doctrine to enable its forward-deployed land forces to launch an armored assault into Pakistani territory on short notice in response to a perceived provocation from Islamabad. This new strategy was devised after the Indian Army’s armored strike corps took three weeks to deploy to the border after the attack on the Indian parliament in 2001, by which time Pakistan had already mobilized its own troops.

Islamabad sees nuclear weapons as its deterrent against a conventional attack, and Cold Start in particular. This is demonstrated by its refusal to adhere to a “No First Use” policy. Pakistan has an extensive plutonium production capacity, and is estimated to possess 130 to 140 warheads, a total that may easily increase to 220 to 250 in a decade, according to a report by the Bulletin of Atomic Scientists.

Many of the new weapons are smaller, short-range tactical weapons intended for targeting frontline troops. To enable a second-strike capability, Pakistan has also empowered local commanders to launch retaliatory nuclear strikes in case the chain of command is disrupted.

While battlefield nuclear weapons are less likely to cause the mass civilian casualties that a strike against a densely populated city would produce, they are deeply worrying in their own way: a state may be more tempted to employ tactical nuclear weapons, and perceive doing so as being intrinsically less risky. However, many simulations of nuclear war suggest that tactical-nuclear-weapon usage rapidly escalates to strategic weapons.

Furthermore, tactical nuclear weapons are necessarily more dispersed, and thus less secure than those stationed in permanent facilities. These issues led the U.S. Army to at first reorganize its tactical nuclear forces in the 1960s, and largely abandon them after the end of the Cold War.

Pakistan fields nearly a dozen different types of missiles to facilitate this strategy, developed with Chinese and North Korean assistance. Ground based tactical systems include the Hatf I, an unguided ground-based rocket with a range of one hundred kilometers, and the Nasr Hatf IX, which can be mounted on mobile quad-launchers. Longer reach is provided by Ghauri II and Shaheen II medium-range ballistic missiles, which can strike targets up to around 1,600 and 2,500 kilometers, respectively.

The Pakistani Air Force’s American-made F-16 fighters are also believed to have been modified to deploy nuclear weapons. The older F-16As and Bs of the Thirty-Eighth Fighter Wing and the newer Cs and Ds of the Thirty-Ninth are both believed to be based near nuclear-weapon storage facilities. The PAF’s five squadrons of Mirage IIIs, based in Karachi and Shorkot, meanwhile, have been modified to launch the domestically-produced Ra’ad nuclear Air-Launched Cruise Missile (ALCM), with a range of 350 kilometers. New JF-17 fighters jointly produced with China are also thought to be capable of carrying the Ra’ad ALCM.

The Pakistani Navy lacks a nuclear strike capability, but appears interested in acquiring one. In January of this year, it released a video claiming to show a test launch of a Babur-3 submarine-launched cruise missile. The domestically produced Babur is similar to the Tomahawk, and designed to approach its target at low altitude to avoid detection. Pakistan already possesses land-based TEL vehicles to deploy the nuclear-capable weapon.

Reflecting its superior conventional abilities, India does adhere to a “No First Use” nuclear weapons policy. Its security posture is also complicated by long lasting tensions with China, dating back to a border war in 1962 in which Beijing seized territory in the Himalayas. Today, China is closely allied economically and militarily with Pakistan, and even has a naval base in Gwadar as part of a strategy to envelop India. India, by contrast, continues to receive much of its weaponry from Russia, but does not enjoy the same kind of military alliance. It has instead dramatically expanded civilian nuclear cooperation with the United States and other nations in the last decades.

India possesses a smaller number of nuclear weapons, estimated in 2015 to range between ninety and 120. However, New Delhi recently acquired a full nuclear triad of air-, land- and sea-based nuclear platforms when it deployed its first home-produced nuclear-powered submarine, the INS Arihant. The Arihant is capable of launching a dozen K-15 Sagarika submarine-launched ballistic missiles. However, these are limited to a range of 750 kilometers, and are thus incapable of reaching the major inland cities of Pakistan or China, a shortcoming India is attempting to address with new K-4 missiles, derived form the land-based Agni-III. New Delhi intends to produce three more nuclear submarines over the years, while Pakistan is considering building one of their own.

India’s chief nuclear arm is thought to lay in its Mirage 2000H and Jaguar fighter-bombers, which can carry nuclear gravity bombs. In 2016, India signed a contract for thirty-six nuclear-capable fourth-generation Rafale fighters from France, further enhancing its aerial striking power. India has also modified its Su-30 fighter-bombers to carry the BrahMos cruise missiles with a range of five hundred kilometers. These could theoretically carry nuclear warheads, though none are believed to have been so equipped so far.

India also has its own array of ground-based nuclear ballistic missiles. The most numerous are slow-firing Prithvi short-range ballistic missiles. Twenty mobile Agni-1 ballistic missiles with a range of seven hundred kilometers are also deployed along the border with Pakistan, while ten heavier Agni-II systems with a range of two thousand kilometers are situated in the northwest for potential strikes on China. India also possesses a small number of rapid-deploying Agni III missiles with a range of 3,500 kilometers, and is developing an Agni IV MRBM and Agni VI ICBM with sufficient range to hit Chinese cities on the Pacific coast.

If there is any silver lining to this steady escalation in nuclear firepower, it’s that neither India nor Pakistan appears to possess chemical or biological weapons. (India completed the destruction of its stock of mustard gas in 2009.) However, the potential for catastrophic loss of human life if nuclear warheads rain down on the cities of the Indian subcontinent is self-evident.

Indian prime minister Narendra Modi and Pakistani prime minister Nawaz Sharif showed goodwill in a surprise meeting in 2015. Unfortunately, neither state appears capable of shaking out of its intractable pattern of conflict, driven by domestic political forces, which makes diplomatic accommodation difficult. The struggle for Kashmir occupies an important part of Pakistani national identity, and there has yet to be a civilian head of state in Islamabad with the will and authority to bring an end to cross-border infiltration and support for terrorist or insurgent fighters. For its part, the Indian Army has failed to respect local Kashmiri leaders and significantly improve its human-rights record.