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The United States maintains nuclear weapons to "deter, dissuade, and defeat" a range of immediate and potential conventional, nuclear, chemical, and biological weapons threats. Still, the United States has pledged not to use nuclear weapons against non-nuclear-weapon states that are members of the nuclear Nonproliferation Treaty (NPT), except if attacked by such a state associated or allied with a nuclear-armed state. At the same time, successive administrations have maintained a policy of "strategic ambiguity" by refusing to rule out nuclear weapons use in response to biological or chemical weapons attacks.
Negative Security Assurances and Nonproliferation
In 1978, the United States first formally made nuclear non-use pledges, also termed "negative security assurances." A 1997 Presidential Decision Directive reaffirmed that pledge, stating:
“The United States reaffirms that it will not use nuclear weapons against non-nuclear-weapon state-parties to the Treaty on the Nonproliferation of Nuclear Weapons, except in the case of an invasion or any other attack on the United States, its territories, its armed forces or other troops, its allies, or on a state toward which it has a security commitment carried out, or sustained by such a non-nuclear-weapon state in association or alliance with a nuclear-weapon state.”
Under the NPT, the five recognized nuclear-weapon states are China, France, Russia, the United Kingdom, and the United States. Three other states, India, Pakistan, and Israel, also possess nuclear weapons and never joined the NPT. After several years of suspected noncompliance with its NPT obligations, North Korea announced its withdrawal from the treaty in 2003 and then three years later conducted its first nuclear test.
In 1995, UN Security Council Resolution 984 acknowledged a U.S. negative security pledge issued by Secretary of State Warren Christopher and similar statements by the governments of the other four NPT nuclear-weapon states. At the 1995 NPT review and extension conference, these negative security assurances were incorporated in its final document's "Principles and Objectives for Non-Proliferation and Disarmament," which was vital to securing indefinite extension of the NPT.
Strategic Ambiguity
Despite the "negative security assurances" pledge, the United States has not ruled out the use of nuclear weapons in response to attacks with chemical or biological weapons. The 2001 Nuclear Posture Review (NPR), for example, maintains the possibility that U.S. nuclear forces may be used to counter threats from non-nuclear adversaries. In addition to China, an NPT nuclear-weapon state, the NPR cites five non-nuclear-weapon states (Iran, Iraq, Libya, North Korea, and Syria) as driving “requirements for nuclear strike capabilities.” While all five states were suspected of pursuing nuclear weapons, none of the five at that time had demonstrated possessing such a capability. All five, however, were believed to have biological and/or chemical weapons or programs. The NPR also stated that “nuclear weapons could be employed against targets able to withstand non-nuclear attack, (for example, deep underground bunkers or bio-weapon facilities).”
U.S. government officials also have spoken about using nuclear weapons to respond to an attack employing any so-called weapons of mass destruction (WMD). In a February 2002 statement, Department of State spokesperson Richard Boucher declared:
“Furthermore, the policy says that we will do whatever is necessary to deter the use of weapons of mass destruction against the United States, its allies, and its interests. If a weapon of mass destruction is used against the United States or its allies, we will not rule out any specific type of military response.”
Boucher noted that similar statements had been made repeatedly since the 1970s, specifically citing statements during the 1991 Persian Gulf War and an April 1996 statement by Secretary of Defense William Perry. Speaking about a suspected Libyan chemical weapons facility at Tarhunah, Perry said that "[if] some nation were to attack the United States with chemical weapons, then they would have to fear the consequences of a response from any weapon in our inventory." Perry added, "we could make a devastating response without the use of nuclear weapons, but we would not forswear that possibility." Still, Perry noted, "in every situation that I have seen so far, nuclear weapons would not be required for response."
Signed in September 2002, National Security Presidential Directive 17 took an apparent step toward making nuclear retaliation to the use of any WMD official U.S. policy. The secret directive, portions of which were leaked, stated, “the United States will continue to make clear that it reserves the right to respond with overwhelming force—including potentially nuclear weapons—to the use of WMD against the United States, our forces abroad, and friends and allies.” A subsequent unclassified version, known as the National Strategy to Combat Weapons of Mass Destruction, was not as explicit, substituting “including through resort to all of our options” for “including potentially nuclear weapons.”
U.S. diplomats have tended to emphasize the negative security assurances policy in international forums, such as arms control negotiations, while other U.S. officials have enunciated purposefully ambiguous qualifications of the pledge in response to specific perceived threats from chemical and biological weapons. The United States has resisted repeated calls for negotiations on a legally-binding instrument on negative security assurances at the UN Conference on Disarmament. Acting Assistant Secretary of State Stephen Rademaker told the conference in May 2006 that the United States “sees no need at this time, however, for the negotiation of new multilateral agreements on…negative security assurances.”