India and Pakistan have been rivals since 1947, when the two countries were born from the dissolution of the British Raj in India. The two countries have gone to war four times since then, in 1947, 1965, 1974 and 1999, and been on the brink of war as recently as 2008. The last war, the 1999 Kargil War, was particularly dangerous as both countries were avowed nuclear powers. If a war on the subcontinent went nuclear, how bad could it get?

India tested its first nuclear device, codenamed “Smiling Buddha,” in May 1974. India had been prompted to build nuclear weapons by China, with which it lost a border war in 1962, and which had considerable conventional forces. More importantly however, it had nuclear weapons, and India felt compelled to build its own. The country maintained a moratorium on further tests until May 1998, when it conducted five tests in rapid order, including four fission and one fusion bomb (which was a partial failure).

Today the country has between ninety and 110 nuclear warheads divided among India’s own version of the nuclear triad consisting of nuclear-capable strike aircraft, land-based missiles and the new ballistic-missile submarine INS Arihant. This is designed to give the country a flexible nuclear arsenal capable of surviving a first strike by another nuclear state. India has a No First Use policy, vowing not to use nuclear weapons first in a conflict.

India’s aerial nuclear strike force consists of 272 Su-30MK1 twin engine fighters on order from Russia, sixty-nine MiG-29s and fifty-one Mirage 2000 fighters, at least some of which have likely been modified to carry nuclear weapons. The land-based missile leg of the triad consists of Prithvi tactical ballistic missiles. With a range of ninety-three miles, these could be used against enemy tactical targets such as air bases, artillery concentrations, headquarters sites or supply depots. The Agni 1–5 series of short, medium, intermediate-range and intercontinental ballistic missiles represent both tactical weapons and longer-range systems capable of Pakistan’s own nuclear-weapons sites, cities, ports and other high-value targets.

Finally, India is constructing a fleet of four ballistic-missile submarines led by INS Arihant. Equipped with both short- and medium-range ballistic missiles, Arihant can carry twelve K-15 Sagarika (“Oceanic”) short-range ballistic missiles with maximum range of 434 miles, or alternately, four K-4 medium-range ballistic missiles with a 2,174-mile range. Protected by India’s naval superiority, the Arihant-class submarines will provide a crucial second-strike capability capable of launching a devastating retaliatory barrage.

Pakistan is estimated to have an arsenal of 110 to 130 nuclear weapons, a number that is believed to be steadily growing. Unlike India, Pakistan does not appear to have vastly more powerful thermonuclear weapons, nor does it have a No First Use policy. In 2015 the Carnegie Endowment for International Peace and the Stimson Center estimated Pakistan’s bomb-making capability at twenty devices annually. At such a rate Pakistan could easily become the fourth- or even third-largest nuclear power in the world.

Like India, Pakistan is also developing a “triad” of land-, air- and sea-based nuclear delivery systems. Islamabad is believed to have modified American-built F-16A fighters and possibly French-made Mirage fighters to deliver nuclear bombs. Land-based missile systems are Hatf series of mobile missiles includes the solid fueled Hatf-III (180 miles), solid fueled Hatf-IV (466 miles) and liquid fueled Hatf V (766 miles). An even longer-range missile, Hatf VI (1,242 miles), is probably now entering service. In order to counter threats stationed on the Nicobar and Andaman Islands, Pakistan is also developing the Shaheen III intermediate-range missile, capable of striking targets out to 1,708 miles.

Pakistan is taking a less expensive route to sea-based nuclear deterrence, outfitting existing ships and submarines with the Babur cruise missile instead of building dedicated missile submarines. The latest version, Babur-2, has a range of 434 miles and uses older Terrain Contour Matching and Digital Scene Matching and Area Co-relation navigation technology. Babur-2 is deployed on both land and at sea on ships where it would be more difficult to track down and destroy. A submarine-launched version, Babur-3, was tested in in early 2017 and would be the most survivable of all Pakistani nuclear delivery systems.

What would a nuclear war be like? A nuclear war in South Asia would start out as a conventional war, which might very well be sparked by a cross-border incident. Uncontrolled escalation could lead to conflict between land, sea and air forces on both sides. The inclination would be for the losing side, especially one seeing tank spearheads barreling down on its major cities, to deploy tactical nuclear weapons.

Tactical nuclear weapons, although they might stabilize the front, would bring the risk of all-out nuclear warfare. The side that had been winning, seeing that the losing side had resorted to battlefield nuclear weapons, might very well unleash an all-out nuclear attack to not only destroy the other side’s nukes but decapitate the enemy government to prevent further launches. Targets would include nuclear research centers, known nuclear weapons facilities, air bases where nuclear-armed aircraft are based, military ports and military headquarters. Many of these would be in and around major metropolitan areas, convenient during peacetime, but bound to causing severe civilian casualties in wartime.

As a nuclear exchange goes back and forth the pressure to escalate out of revenge could prove irresistible and lead to the deaths of millions. A successful two-hundred-kiloton thermonuclear device dropped on Islamabad, the kind which India attempted to test in 1998, would kill nearly 225,000 and injure 442,000 more. A fifty-kiloton strike on Karachi would kill eight hundred thousand and injure 1.8 million more.

Meanwhile, Pakistani nuclear weapons used on India, although smaller in explosive yield, would create even more devastating results: a fifty-kiloton bomb dropped on New Delhi would kill 468,000 and injure 1.5 million. The same bomb dropped on Mumbai would cause a similar number of casualties, and dropped on Jaipur would kill and injure 1.3 million. The damage on both sides would be so catastrophic that local governments would be overwhelmed, risking loss of control over both societies.

Neither side wants war. The danger that one of the many border incidents between the two countries could spiral out of control into a conventional war and then a nuclear one. While in wars past one country simply lost a war, a nuclear war would ensure that both sides would lose, killing millions and setting back both countries decades. An agreement by both sides to limit their numbers and use is crucial. Once nuclear weapons are used there is no sure path to victory—or even mere survival.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

Image: Akula-class submarine underway. Wikimedia Commons/U.S. Navy