Key point: Even though only India and Pakistan would be destroyed directly by nuclear weapons, the radiation and fallout would change the world. If enough weapons were used, it is very likely that the sun would be blocked out causing crop failures and mass starvation.

Sandwiched between Iran, China, India and Afghanistan, Pakistan lives in a complicated neighborhood with a variety of security issues. One of the nine known states known to have nuclear weapons, Pakistan’s nuclear arsenal and doctrine are continually evolving to match perceived threats. A nuclear power for decades, Pakistan is now attempting to construct a nuclear triad of its own, making its nuclear arsenal resilient and capable of devastating retaliatory strikes.

Pakistan’s nuclear program goes back to the 1950s, during the early days of its rivalry with India. President Zulfikar Ali Bhutto famously said in 1965, “If India builds the bomb, we will eat grass or leaves, even go hungry, but we will get one of our own.”

The program became a higher priority after the country’s 1971 defeat at the hands of India, which caused East Pakistan to break away and become Bangladesh. Experts believe the humiliating loss of territory, much more than reports that India was pursuing nuclear weapons, accelerated the Pakistani nuclear program. India tested its first bomb, codenamed “Smiling Buddha,” in May 1974, putting the subcontinent on the road to nuclearization.

Pakistan began the process of accumulating the necessary fuel for nuclear weapons, enriched uranium and plutonium. The country was particularly helped by one A. Q. Khan, a metallurgist working in the West who returned to his home country in 1975 with centrifuge designs and business contacts necessary to begin the enrichment process. Pakistan’s program was assisted by European countries and a clandestine equipment-acquisition program designed to do an end run on nonproliferation efforts. Outside countries eventually dropped out as the true purpose of the program became clear, but the clandestine effort continued.

Exactly when Pakistan had completed its first nuclear device is murky. Former president Benazir Bhutto, Zulfikar Bhutto’s daughter, claimed that her father told her the first device was ready by 1977. A member of the Pakistan Atomic Energy Commission said design of the bomb was completed in 1978 and the bomb was “cold tested”—stopping short of an actual explosion—in 1983.

Benazir Bhutto later claimed that Pakistan’s bombs were stored disassembled until 1998, when India tested six bombs in a span of three days. Nearly three weeks later, Pakistan conducted a similar rapid-fire testing schedule, setting off five bombs in a single day and a sixth bomb three days later. The first device, estimated at twenty-five to thirty kilotons, may have been a boosted uranium device. The second was estimated at twelve kilotons, and the next three as sub-kiloton devices.

The sixth and final device appears to have also been a twelve-kiloton bomb that was detonated at a different testing range; a U.S. Air Force “Constant Phoenix” nuclear-detection aircraft reportedly detected plutonium afterward. Since Pakistan had been working on a uranium bomb and North Korea—which shared or purchased research with Pakistan through the A. Q. Khan network—had been working on a uranium bomb, some outside observers concluded the sixth test was actually a North Korean test, detonated elsewhere to conceal North Korea’s involvement although. There is no consensus on this conclusion.

Experts believe Pakistan’s nuclear stockpile is steadily growing. In 1998, the stockpile was estimated at five to twenty-five devices, depending on how much enriched uranium each bomb required. Today Pakistan is estimated to have an arsenal of 110 to 130 nuclear bombs. In 2015 the Carnegie Endowment for International Peace and the Stimson Center estimated Pakistan’s bomb-making capability at twenty devices annually, which on top of the existing stockpile meant Pakistan could quickly become the third-largest nuclear power in the world. Other observers, however, believe Pakistan can only develop another forty to fifty warheads in the near future.

Pakistani nuclear weapons are under control of the military’s Strategic Plans Division, and are primarily stored in Punjab Province, far from the northwest frontier and the Taliban. Ten thousand Pakistani troops and intelligence personnel from the SPD guard the weapons. Pakistan claims that the weapons are only armed by the appropriate code at the last moment, preventing a “rogue nuke” scenario.

Pakistani nuclear doctrine appears to be to deter what it considers an economically, politically and militarily stronger India. The nuclear standoff is exacerbated by the traditional animosity between the two countries, the several wars the two countries have fought, and events such as the 2008 terrorist attack on Mumbai, which were directed by Pakistan. Unlike neighboring India and China, Pakistan does not have a “no first use” doctrine, and reserves the right to use nuclear weapons, particularly low-yield tactical nuclear weapons, to offset India’s advantage in conventional forces.

Pakistan currently has a nuclear “triad” of nuclear delivery systems based on land, in the air and at sea. Islamabad is believed to have modified American-built F-16A fighters and possibly French-made Mirage fighters to deliver nuclear bombs by 1995. Since the fighters would have to penetrate India’s air defense network to deliver their payloads against cities and other targets, Pakistani aircraft would likely be deliver tactical nuclear weapons against battlefield targets.

Land-based delivery systems are in the form of missiles, with many designs based on or influenced by Chinese and North Korean designs. The 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). The CSIS Missile Threat Initiative believes that as of 2014, Hatf VI (1242 miles) is likely in service. Pakistan is also developing a Shaheen III intermediate-range missile capable of striking targets out to 1708 miles, in order to strike the Nicobar and Andaman Islands.

The sea component of Pakistan’s nuclear force consists of the Babur class of cruise missiles. The latest version, Babur-2, looks like most modern cruise missiles, with a bullet-like shape, a cluster of four tiny tail wings and two stubby main wings, all powered by a turbofan or turbojet engine. The cruise missile has a range of 434 miles. Instead of GPS guidance, which could be disabled regionally by the U.S. government, Babur-2 uses older Terrain Contour Matching (TERCOM) and Digital Scene Matching and Area Co-relation (DSMAC) navigation technology. Babur-2 is deployed on both land and at sea on ships, where they would be more difficult to neutralize. A submarine-launched version, Babur-3, was tested in January and would be the most survivable of all Pakistani nuclear delivery systems.

Pakistan is clearly developing a robust nuclear capability that can not only deter but fight a nuclear war. It is also dealing with internal security issues that could threaten the integrity of its nuclear arsenal. Pakistan and India are clearly in the midst of a nuclear arms race that could, in relative terms, lead to absurdly high nuclear stockpiles reminiscent of the Cold War. It is clear that an arms-control agreement for the subcontinent is desperately needed.

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. This first appeared several years ago and is being republished due to reader interest.

Image: Reuters.