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Weapons of Mass Destruction (WMD)

United States Nuclear Weapons

After a comprehensive review of our nuclear forces, the President determined that the US could ensure the security of the United States and our allies and partners and maintain a strong and credible strategic deterrent while safely pursuing up to a one-third reduction in deployed strategic nuclear weapons from the level established in the New START Treaty. The US intent was to seek negotiated cuts with Russia so that it could continue to move beyond Cold War nuclear postures.

Nuclear warheads are identified by a W with a program number, e.g. W88. Nuclear bombs are identified by a B with a program number, e.g. B61. Modifications include Mod numbers, e.g., B61, Mod 11, or simply B61-11.

Early History

The atomic bomb, originated in 1939. At that time, Dr. Albert Einstein persuaded President Franklin Roosevelt to fund development of the bomb. Amid extreme secrecy, scientists from around the United States worked day and night to make the concept a reality. Finally, by late 1943, the scientists were confident enough to tell the Army Air Forces (AAF) to begin preparing for the bomb's use. On 06 August 1945 the B-29 "Enola Gay" flew from Tinian to strike Hiroshima, Japan, on the world's first atomic bombing mission. Three days later "Bockscar" dropped the plutonium bomb on Nagasaki. The Japanese surrendered in the following days thereby ending World War II.

At first the United States relied on US Strategic Air Command to defend Europe against Soviet attack, but in 1950 the start of the Korean War led to European re-equipping and consideration of the deployment of tactical nuclear weapons. By 1951, progress on a thermonuclear bomb of smaller dimensions revived interest in the long-range ballistic missile. Two months before President Truman announced that the United States would develop the thermonuclear bomb, the Air Force contracted with Consolidated Vultee Aircraft Corporation (later Convair) to resume study, and then to develop, the Atlas intercontinental ballistic missile, a project that had been dormant for four years.

Studies of the possibilities of using thermonuclear reactions to obtain very large explosions began in the summer of 1942-almost a year before the Los Alamos Laboratory was formed. Such studies continued here during the war, though at a necessarily modest rate partly because the Laboratory's primary mission was to develop a fission bomb as rapidly as possible, partly because a fission bomb appeared to be prerequisite to the initiation of any thermonuclear reaction, and partly because the theoretical investigation of the feasibility of achieving a large-scale thermonuclear re-action- at least the "Classical Super" form then considered-was enormously more difficult than that required in connection with obtaining an explosive fission reaction.

Studies of possible thermonuclear weapons continued there in the years immediately after the war, but these too were necessarily limited in scope. Only one of the small but capable group working on the Super during the war continued on the Los Alamos staff after the spring of 1946. In addition, the need for improvements in fission weapons was evident and pressing. And, for several years at least, the computing resources available here (or anywhere else in the country) were completely inadequate for a definitive handling of the problems posed by a thermo-nuclear weapon.

Nevertheless, in 1947 the pattern emerged for a possible "booster," that is, a device in which a small amount of thermonuclear fuel is ignited by a fission reaction and produces neutrons that in turn enhance the fission reaction. In 1948, it was decided to include a test of such a system in the series then planned for 1951. Following the first test of a fission bomb by the Soviets in August 1949, President Truman decided at the end of January 1950 that the United States should undertake a concerted effort to achieve a thermonuclear weapon even though no clear and persuasive pattern for such a device was available at that time. In May of 1951, as part of the Greenhouse test series, two experiments involving thermonuclear reactions were conducted. One, the George shot, the design of which resulted from the crash program on the H-bomb, confirmed that our understanding of means of initiating a small-scale thermonuclear reaction was adequate. The other, the Item shot, demonstrated that a booster could be made to work.

Quite fortuitously, in the period between one and two months preceding these experiments but much too late to have any effect on their designs, a new insight concerning thermonuclear weapons was realized. Almost immediately this insight gave promise of a feasible approach to thermonuclear weapons, provided only that the design work be done properly. This approach was the one of which Robert Oppenheimer was later (1954) to say, "The program we had in 1949 was a tortured thing that you could well argue did not make a great deal of technical sense . . . . The program in 1951 was technically so sweet that you could not argue about that." On this new basis and in an impressively short time, considering the amount and novelty of the design work and engineering required, the Mike shot, with a yield of about 10 megatons, was conducted in the Pacific on November 1, 1952.

As tested, Mike was not a usable weapon: it was quite large and heavy, and its thermonuclear fuel, liquid deuterium, required a refrigeration plant of great bulk and complexity. Nevertheless, its performance amply confirmed the validity of the new approach. In the spring of 1954, a number of devices using the new pattern were tested, including the largest nuclear explosion (about 15 megatons) ever conducted by the United States. Some of these devices were readily adaptable (and adopted) for use in the stockpile.

After 1954, a large number of thermonuclear tests were carried out combining and improving the features first demonstrated in the Item and Mike shots. The continuing objective had been weapons of smaller size and weight, of improved efficiency, more convenient and safe in handling and delivery, and more specifically adapted to the needs of new missiles and aircraft.

The early atomic bomb required polonium/beryllium detonator pits (or initiators) to generate the neutrons of the explosive sequence. Polonium-210 has a half-life of about 138 days, a fact that mandated the replacement of the pits periodically. In order to access the pits, personnel opened threaded couplings machined from fissile uranium-a process that produced radioactive waste items. With the phasing out of the atomic bomb, and the phasing in of the TN weapon, a sealed neutron initiator replaced the polonium/beryllium pit. These second generation capsules, brought into the inventory as of late 1954, still required periodic disassembly to verify the integrity of the fissile materials. As of 1962, capsules were completely phased out.

In 1953, Eisenhower introduced his new look strategy which came to mean that any aggression by USSR would be met by a massive retaliatory response using nuclear weapons. Between 1945 and 1962, during the atmospheric test series, the US Government conducted 235 nuclear weapons tests, principally in Nevada and the Pacific. Stockpiling of the atomic bomb began slowly, with only 13 in the entire arsenal in 1947; 56 in 1948; 298 in mid-1950. The leap came during the Korean war, between 1950 and the close of 1952, when stockpiles reached a total of 832 bombs. In 1955, the United States sustained an inventory of 2,280 nuclear (atomic and thermonuclear) bombs.

With President Kennedy in power the old "trip-wire" strategy which could have resulted in massive nuclear war was replaced in 1961 by "flexible response." The number of nuclear warheads in strategic alert forces increased from 850 on 30 June 30 1961 to 2,700 estimated as of 30 June 1965.

By the 1990s, the most modern safety features available at the time in US nuclear weapons were incorporated in the Peacekeeper intercontinental ballistic missile warhead (W87), the ground-launched cruise missile warhead (W84), and a modern strategic bomb (the B83)-all first deployed in the 1980s. They included features such as high explosive that was virtually impossible to detonate inadvertently (developed by Los Alamos and Livermore in the 1970s), as well as creative features that enhanced electrical nuclear detonation safety and make the weapons safe in the event of fire.

Force Reductions

Moscow Treaty

The Moscow Treaty was signed at Moscow on May 24, 2002, and entered into force on June 1, 2003. The Moscow Treaty both reflected and significantly contributed to the emergence of the new strategic relationship between the United States and Russia. The Treaty places upon the United States a legal obligation to implement fully its publicly announced plans to reduce to a level of 1,700 to 2,200 strategic nuclear warheads by December 31, 2012.

In using the term "operationally deployed strategic nuclear warheads" the United States meant reentry vehicles on ICBMs in their launchers, reentry vehicles on SLBMs in their launchers on board submarines, and nuclear armaments loaded on heavy bombers or stored in weapons storage areas of heavy bomber bases. A small number of spare strategic nuclear warheads (including spare ICBM warheads) were located at heavy bomber bases and the United States did not consider these warheads to be operationally deployed strategic nuclear warheads.

The Treaty made clear that the Parties need not implement their reductions in an identical manner. Russia, like the United States, might reduce its strategic nuclear warheads by any method it chose. It was not immediately clear how Russia intended to count its reductions for purposes of the Moscow Treaty. It could use "operationally deployed strategic nuclear warheads" or some other method. Russia might or might not implement Moscow Treaty reductions in the same way it has implemented reductions under START. Moscow Treaty numbers were not comparable to START Treaty data due to the different counting approaches of the two treaties.

The US plan for achieving by December 31, 2012, the strategic offensive reductions required by Article I of the Treaty involved, as the first planned step in reducing U.S. operationally deployed strategic nuclear warheads:

  • retiring 50 Peacekeeper ICBMs,
  • removing four Trident submarines from strategic service, and
  • no longer maintaining the ability to return the B-1B heavy bomber to nuclear service.

These steps were already underway (Peacekeeper deactivation and SSBN-to-SSGN modification) or completed (B-1B) at the time the treaty came into force. At any given time, the United States would have two of the remaining 14 Trident SSBNs in overhaul. Those SSBNs in overhaul would not contain operationally deployed strategic nuclear warheads.

The United States planned to reduce its operationally deployed strategic nuclear warheads to 3,500 to 4,000 by 2007. Specific decisions about US forces beyond 2007 had not been made at the time the treaty came into force. It was anticipated that reductions beyond 2007 would involve decreasing the number of operationally deployed strategic nuclear warheads on ballistic missiles and lowering the number of operationally deployed warheads at heavy bomber bases. These plans, however, would be periodically assessed, and would evolve over time.


On 8 April 2010, President Obama and President Medvedev of Russia signed the Treaty on Measures for the Further Reduction and Limitation of Strategic Offensive Arms also known as the New START Treaty and its protocol.

Strategic Offensive Reductions: Under the Treaty, the U.S. and Russia were to meet the Treaty’s central limits on strategic arms by February 5, 2018; seven years from the date the Treaty entered into force. While each Party has the flexibility to determine for itself the structure of its strategic forces within the aggregate limits of the Treaty, these limits are based on the rigorous analysis conducted by Department of Defense planners in support of the 2010 Nuclear Posture Review.

Aggregate limits:

  • 1,550 warheads. Warheads on deployed ICBMs and deployed SLBMs count toward this limit and each deployed heavy bomber equipped for nuclear armaments counts as one warhead toward this limit.
  • A combined limit of 800 deployed and non-deployed ICBM launchers, SLBM launchers, and heavy bombers equipped for nuclear armaments.
  • A separate limit of 700 deployed ICBMs, deployed SLBMs, and deployed heavy bombers equipped for nuclear armaments.

Verification and Transparency: The Treaty has a verification regime that combines the appropriate elements of the 1991 START Treaty with new elements tailored to the limitations of the Treaty. Measures under the Treaty include on-site inspections and exhibitions, data exchanges and notifications related to strategic offensive arms and facilities covered by the Treaty, and provisions to facilitate the use of national technical means for treaty monitoring. To increase confidence and transparency, the Treaty also provides for the exchange of telemetry.

Treaty Terms: The Treaty’s duration is ten years, unless superseded by a subsequent agreement. The Parties may agree to extend the Treaty for a period of no more than five years. The Treaty includes a withdrawal clause that is standard in arms control agreements. The 2002 Moscow Treaty terminated when the New START Treaty entered into force.

No Constraints on Missile Defense and Conventional Strike: The Treaty does not contain any constraints on testing, development or deployment of current or planned U.S. missile defense programs or long-range conventional strike capabilities.

2010 Nuclear Posture Review

The 2010 Nuclear Posture Review report focused on five objectives of the U.S. nuclear weapons policies and posture:

1) Preventing nuclear proliferation and nuclear terrorism: The NPR placed preventing nuclear proliferation and nuclear terrorism atop the U.S. nuclear agenda by defining specific steps to strengthen the global non-proliferation regime, and accelerating the securing of nuclear materials worldwide. It also renewed the U.S. commitment to hold fully accountable any state, terrorist group, or other nonstate actor that supports or enables terrorist efforts to obtain or use weapons of mass destruction, whether by facilitating, financing, or providing expertise or safe haven for such efforts.

2) Reducing the role of U.S. nuclear weapons in U.S. national security strategy: The U.S. declaratory policy was updated so that the United States would not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the Nuclear Non-Proliferation Treaty (NPT) and in compliance with their nuclear nonproliferation obligations. The United States would also only consider the use of nuclear weapons in extreme circumstances to defend the vital interests of the United States or its allies and partners, and would continue to strengthen conventional capabilities and reduce the role of nuclear weapons in deterring non-nuclear attacks, with the objective of making deterrence of nuclear attack on the United States or its allies and partners the sole purpose of U.S. nuclear weapons.

3) Maintaining strategic deterrence and stability at reduced nuclear force levels: As reflected in the New START agreement, the United States and Russia agreed to limits of 1,550 accountable strategic warheads, 700 deployed strategic delivery vehicles, and 800 deployed and non-deployed strategic launchers; with the U.S. nuclear Triad of ICBMs, SLBMs, and nuclear-capable heavy bombers maintained. All U.S. ICBMs were to be “de-MIRVed” to a single warhead each. The United States would also pursue post-New START arms control with Russia to address not only strategic weapons, but also non-strategic and non-deployed nuclear weapons. Similarly, the NPR called for the United States to pursue high-level bilateral dialogues with Russia and China aimed at promoting more stable and transparent strategic relationships.

4) Strengthening regional deterrence and reassuring U.S. allies and partners: Under this axis, the U.S. would keep pursuing a comprehensive approach to broaden regional security architectures, including through missile defenses and improved conventional forces, though maintaining the need for deterrence to require a nuclear component so long as regional nuclear threats to US forces, allies, and partners remained. As a result, the NPR called for the United States to retain the capability to forward-deploy U.S. nuclear weapons on tactical fighter-bombers and heavy bombers while retiring nuclear-tipped, sea-launched cruise missile (TLAM-N) deemed redundant in the overall mix of capabilities.

5) Sustaining a safe, secure, and effective nuclear arsenal: The 2010 NPR called for the U.S. to modernize the nuclear weapons infrastructure, sustain the science, technology, and engineering base, invest in human capital, and ensure senior leadership focus. The significantly increased investments called for in the NPR were to not only guarantee the U.S. stockpile, but facilitate further nuclear reductions, and help enhance the ability to stem nuclear proliferation and nuclear terrorism. It would also extend the life of warheads currently in the nuclear arsenal and provide an alternative to developing new nuclear weapons. This effort is to be guided by a number of principles, namely:

  • The United States will not conduct nuclear testing, and will seek ratification and entry into force of the Comprehensive Nuclear Test Ban Treaty.
  • The United States will not develop new nuclear warheads. Life Extension Programs (LEPs) will use only nuclear components based on previously tested designs, and will not support new military missions or provide for new military capabilities.
  • The Administration will study options for ensuring the safety, security, and reliability of nuclear warheads on a case-by-case basis, consistent with the congressionally mandated Stockpile Management Plan. The full range of LEP approaches will be considered: refurbishment of existing warheads, reuse of nuclear components from different warheads, and replacement of nuclear components.
  • In any decision to proceed to engineering development for warhead LEPs, the Administration will give strong preference to options for refurbishment or reuse. Replacement of nuclear components would be undertaken only if critical Stockpile Management Program goals could not otherwise be met, and if specifically authorized by the President and approved by Congress.

Obama's new guidance

On 19 June 2013, President Obama announced new guidance that aligned US nuclear policies to the 21st century security environment. This was the latest in a series of concrete steps the President had made to advance his Prague agenda and the long-term goal of achieving the peace and security of a world without nuclear weapons. Following the release of the 2010 Nuclear Posture Review and ratification of the New START Treaty, the President directed the Department of Defense, the Department of State, Department of Energy, and the intelligence community, to conduct a detailed analysis of US nuclear deterrence requirements and policy in order to ensure US nuclear posture and plans were aligned to address the existing security environment. This review was based on the principle that a robust assessment of the security environment and resulting Presidential guidance had to drive nuclear employment planning, force structure, and posture decisions.

After a comprehensive review of US nuclear forces, the President determined that the US could ensure the security of the United States and our allies and partners and maintain a strong and credible strategic deterrent while safely pursuing up to a one-third reduction in deployed strategic nuclear weapons from the level established in the New START Treaty. The US intent was to seek negotiated cuts with Russia so that it could continue to move beyond Cold War nuclear postures.

The analysis leading to the June 2013 announcement did not set out to address weapons forward deployed in Europe in support of NATO. The role of nuclear weapons in NATO was examined as part of the last year’s Deterrence and Defense Posture Review, which affirmed Allies’ support for further U.S.-Russian nuclear reductions, and underscored that any changes in NATO’s nuclear posture must be an Alliance decision. As the US continued to implement the NPR, it remained focused on maintaining and improving strategic stability with both Russia and China.

In summation, President Obama’s new guidance announced on 19 June 2013:

  • affirmed that the United States will maintain a credible deterrent, capable of convincing any potential adversary that the adverse consequences of attacking the United States or our allies and partners far outweigh any potential benefit they might seek to gain through an attack.
  • directed DOD to align U.S. defense guidance and military plans with the policies of the NPR, including that the United States will only consider the use of nuclear weapons in extreme circumstances to defend the vital interests of the United States or its allies and partners. The guidance narrowed U.S. nuclear strategy to focus on only those objectives and missions that are necessary for deterrence in the 21st century. In so doing, the guidance took further steps toward reducing the role of nuclear weapons in our security strategy.
  • directed DOD to strengthen non-nuclear capabilities and reduce the role of nuclear weapons in deterring non-nuclear attacks.
  • directed DOD to examine and reduce the role of launch under attack in contingency planning, recognizing that the potential for a surprise, disarming nuclear attack was exceedingly remote. While the United States would retain a launch under attack capability, DOD would focus planning on the more likely 21st century contingencies.
  • codified an alternative approach to hedging against technical or geopolitical risk, which would lead to more effective management of the nuclear weapons stockpile.
  • reaffirmed that as long as nuclear weapons existed, the United States would maintain a safe, secure, and effective arsenal that guaranteed the defense of the U.S. and its allies and partners. President Obama supported significant investments to modernize the nuclear enterprise and maintain a safe, secure, and effective arsenal. The administration would continue seeking congressional funding support for the enterprise.

The FY 2016 Stockpile Stewardship and Management Plan [SSMP] buildt upon last year’s SSMP in balancing NNSA’s investments in maintaining and modernizing the stockpile, revitalizing its physical infrastructure and workforce, and sustaining its research, development, testing, and evaluation programs. The joint DOE and Department of Defense “3+2 Strategy” for stockpile modernization through life extension programs implemented by this SSMP provides the opportunity to continue the Administration’s commitment to maintaining a safe, secure, and effective deterrent while reducing the size of the stockpile.

For example, once the B61-12 life extension program is completed in FY 2025 and confidence is gained in the B61-12 weapons in service, the B83-1, the last megaton-class weapon in America’s nuclear arsenal, would be retired. The combination of these actions would result in a 50 percent reduction in the number of nuclear gravity bombs in the stockpile, the removal of a megaton-class weapon, an 80 percent reduction in amount of special nuclear material in the bomb portion of the air leg, and a reduction in the safety and security risks associated with the stockpile.

Furthermore, consistent with life extension program plans, NNSA’s investments in plutonium would allow war-reserve-quality production of 30 plutonium pits per year by FY 2026 and 50 to 80 pits per year by 2030. Similarly, NNSA's Uranium Strategy would allow recapitalizing critical enriched uranium operations that currently reside in outdated facilities. These plans for these essential capabilities are consistent with the requirements outlined in the FY 2015 National Defense Authorization Act.

To support this plan, NNSA for the sixth consecutive year increased the budget request for Weapons Activities. If adopted by Congress, this budget request would increase funding by $891 million from the comparable FY 2015 enacted level. Much of this 11.2 percent increase would be devoted to stockpile life extension programs and recapitalization of critical plutonium and uranium capabilities. In addition, the schedule for the W80-4 life extension program has been moved up 2 years, and a conventional high-explosive refresh has been added to the W88 Alteration.


Eight of the nine weapons in the stockpile by the 1990s were not as safe and secure as they could be made. Only the W84 nuclear warhead was equipped with all of the safety and surety features available. The other eight designs did not incorporate all of the safety and surety features that were available. The W62, in fact, did not have any safety features.

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