Understanding Nuclear Threats
Understanding the Nuclear Weapons Threat
While it has been more than twenty years since the end of the Cold War, the existence of thousands of nuclear weapons continues to pose a serious global threat. The likelihood of a nuclear war between the United States and Russia has decreased, but the continued presence of large stockpiles makes the accidental or unauthorized use of nuclear weapons a persistent risk. Many of the countries with smaller nuclear arsenals, such as India and Pakistan, are actively engaged in regional conflicts, making the possibility of regional nuclear war a concern. North Korea illicitly acquired nuclear weapons, and other countries, including Iran and Syria, have violated their nuclear safeguards commitments and are suspected of covertly pursuing nuclear weapons capabilities. In the post-9/11 world, the potential for catastrophic nuclear terrorism is also a serious threat. A number of efforts by governments, international organizations, and non-governmental organizations are underway to attempt to mitigate the nuclear threat—but significantly reducing the risk of nuclear weapons use will require the sustained long-term commitment of the entire international community.
Who Has Nuclear Weapons?
Nine countries are known or widely considered to possess nuclear weapons: China, France, India, Israel, North Korea, Pakistan, Russia, the United Kingdom, and the United States. It is estimated these nine countries collectively hold over 17,300 nuclear warheads.  The United States and Russia possess approximately 94% of the world’s nuclear weapons.  While the possibility of intentional nuclear war has greatly decreased since the end of the Cold War, ongoing regional tensions between nuclear-armed countries such as India and Pakistan pose a continued risk in this regard. Furthermore, the continued existence of large deployed nuclear arsenals in many of the nuclear possessing states poses risks of accidental or unauthorized use. However, disarmament progress is not historically unprecedented. South Africa voluntarily disclosed and dismantled its nuclear weapons program, and following the dissolution of the Soviet Union, Belarus, Kazakhstan, and Ukraine voluntarily transferred the Soviet nuclear weapons on their territories to Russia and joined the NPT as non-nuclear weapon states.
What is a Nuclear Weapon, and Why is it Different from a Conventional Weapon?
Often referred to as weapons of mass destruction, nuclear weapons are fundamentally different from conventional weapons due to their potential, if used in sufficient numbers, to literally destroy life on earth. A nuclear weapon is an explosive device which relies on nuclear rather than chemical reactions, allowing it to harness a far greater amount of energy than a conventional explosive. For example, the W87, a modern U.S. nuclear warhead, has an explosive yield of 300 kilotons, which is equivalent to 300,000 tons of TNT.  The nuclear reactions integral to nuclear weapons can be derived from fission, or a combination of fission and fusion (called a thermonuclear weapon). A sufficient amount of fissile material, such as highly enriched uranium or plutonium, is required to construct a nuclear weapon. The destructive power of a nuclear weapon comes from the blast (pressure shock wave), thermal radiation (heat), and nuclear radiation (prompt and delayed).  Because the production of fissile materials is a complex process requiring extensive resources, efforts to secure global stocks of highly enriched uranium and plutonium against theft or diversion are key to reducing the threats of nuclear terrorism and proliferation.
Testing, Delivery, and Use of Nuclear Weapons
Countries with nuclear weapons have conducted approximately 2,055 known tests,  but nuclear weapons have only been used twice in warfare, both times during World War II. On 6 August 1945 the United States dropped an atomic bomb on the Japanese city of Hiroshima, and three days later dropped a second bomb on Nagasaki, Japan. To deliver nuclear weapons, a country’s war plans typically rely on either bomber aircraft (the oldest and least reliable delivery system), or ballistic missiles (which may be launched from land-based silos or continuously patrolling submarines). Non-state actors without an advanced delivery system could physically transport an improvised nuclear device (IND) to the target site, or detonate a nuclear device at the place of assembly.
How is the Peaceful Use of Nuclear Technologies Linked with Proliferation?
As more countries invest in civilian nuclear infrastructure, particularly nuclear power, the trade in dual-use goods (which can be used for peaceful or military purposes), increases. Nuclear power reactors, while they provide an important source of energy, also produce plutonium in their spent fuel that could potentially be used for weapons purposes. Some research reactors and medical isotope producers continue to rely on the use of highly enriched uranium, which is a weapons-usable material. Determining how to encourage the peaceful use of nuclear technologies while preventing the further proliferation of nuclear weapons is therefore an ongoing policy challenge.
How is the International Community Working to Reduce the Nuclear Threat?
Recognizing the dangers both of existing nuclear arsenals and of new countries acquiring nuclear weapons, the international community has established several treaties and regimes dedicated to nonproliferation and disarmament. The 1968 Treaty on the Non-Proliferation of Nuclear Weapons (NPT) is the cornerstone of these efforts. The NPT recognizes five nuclear weapon states (France, China, Russia, the United Kingdom, and the United States), who are obliged to work towards “general and complete disarmament,” and 184 non-nuclear weapon states, who must refrain from pursuing nuclear weapons, but are permitted to possess peaceful nuclear programs if sensitive activities are safeguarded by the International Atomic Energy Agency (IAEA). India, Pakistan, and Israel have never joined the NPT, and North Korea ratified the NPT in 1985, but unilaterally withdrew in 2003 before testing nuclear devices in 2006, 2009, and 2013. Both Syria and Iran have undertaken questionable nuclear activities and been referred to the United Nations Security Council due to judgments of noncompliance with their IAEA safeguards agreements.
Reducing the Threat of Nuclear Terrorism
The possibility that non-state actors might steal or illicitly purchase highly enriched uranium or plutonium and use them to construct an improvised nuclear device remains an ongoing concern. While the 9/11 terrorist attacks gave policymakers a renewed sense of urgency, the need for nuclear threat reduction efforts first rose to prominence following the collapse of the Soviet Union (USSR) in 1991. Concerned that political and economic instability in the former USSR would lead to the theft or illicit trafficking of nuclear materials or weapons, U.S. Senators Sam Nunn and Richard Lugar founded the Cooperative Threat Reduction (CTR) Program to aid the successor states with nuclear security. The United States subsequently expanded its threat reduction efforts under programs such as the Global Threat Reduction Initiative, which is working to reduce the civil use of HEU globally. In 2003 revelations that the A.Q. Khan network had illicitly sold critical nuclear technologies to North Korea and other states of proliferation concern highlighted serious gaps in international export controls.  Initiatives such as UNSCR 1540, which requires all states to implement measures aimed at preventing non-state actors from acquiring NBC weapons, related materials, and their means of delivery, aim to fill this gap. In 2009, the U.S. Obama administration announced efforts to lock down all nuclear weapons-usable materials in four years, convening a multilateral Nuclear Security Summit attended by 47 countries in 2010 to advance this goal. A follow-on summit occurred in Seoul, South Korea in 2012, with a third scheduled for 2014 in the Netherlands. While all of these programs have made significant progress in reducing the nuclear terrorism threat, continued and enhanced international cooperation will be required to succeed in keeping nuclear materials out of the hands of terrorists.
Progress Toward Nuclear Disarmament
Multilateral negotiations on legally-binding nuclear disarmament measures have proven difficult, as demonstrated by the 15-year stalemate at the Conference on Disarmament. However, there has been some positive progress in the disarmament sphere, with renewed U.S.-Russia bilateral arms reductions occurring under the New START Treaty, and signs of greater political commitment to the disarmament goal by the United States and others in the international community. In 2007, four senior U.S. statesmen – George Shultz, William Perry, Henry Kissinger, and Sam Nunn – set forth the vision of a world free of nuclear weapons in an op-ed for the Wall Street Journal.  The op-ed and subsequent related work lent renewed momentum to nuclear disarmament debates in the United States and other parts of the world. While U.S. President Barack Obama remains committed to maintaining the U.S. nuclear arsenal in the interim, he appears to share the “four horsemen’s” long-term disarmament goal, stating in his famous April 2009 Prague Speech, “today, I state clearly and with conviction America's commitment to seek the peace and security of a world without nuclear weapons.” 
 "World Nuclear Weapons Stockpile Report," Ploughshares Fund, 4 March 2013, www.ploughshares.org.
 "World Nuclear Weapons Stockpile Report," Ploughshares Fund, 4 March 2013, www.ploughshares.org.
 Robert S. Norris and Hans M. Kristensen, “U.S. nuclear forces, 2013,” Bulletin of the Atomic Scientists, March/April 2013, p. 80, www.thebulletin.org.
 S. Glasstone and P.J. Dolan, The Effects of Nuclear Weapons (Washington, D.C.: U.S. Government Printing Office, 1977).
 "Nuclear Testing 1945-Today," Preparatory Commission for the Comprehensive Test Ban Treaty Organization, 2013, www.ctbto.org. North Korea is the only country to have conducted a nuclear test since 1998, having done so on three occasions in 2006, 2009, and 2013 respectively.
 David Albright and Corey Hinderstein, “Unraveling the A.Q. Khan and Future Proliferation Networks,” The Washington Quarterly 28:2, Spring 2005, pp. 111-128.
 George P. Shultz, William J. Perry, Henry A. Kissinger and Sam Nunn, “A World Free of Nuclear Weapons,” The Wall Street Journal, 4 January 2007.
 The White House: Office of the Press Secretary, "Remarks by President Barack Obama," Hradcany Square: Prague, Czech Republic, 5 April 2009, www.whitehouse.gov.