United States of America
Practice Relating to Nuclear Weapons
The US Naval Handbook (2007) states:
There are no rules of customary or conventional international law prohibiting nations from employing nuclear weapons in armed conflict. In the absence of such an express prohibition, the use of nuclear weapons against enemy combatants and other military objectives is not unlawful. Employment of nuclear weapons is, however, subject to the following principles: the right of the parties to the conflict to adopt means of injuring the enemy is not unlimited; it is prohibited to launch attacks against the civilian population as such; and distinction must be made at all times between combatants and civilians to the effect that the latter be spared as much as possible. Given their destructive potential, the decision to authorize employment of nuclear weapons should emanate from the highest level of government. For the United States, that authority resides solely with the President. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 10.2.1.
In 1996, during a debate in the First Committee of the UN General Assembly, the United States stated:
The United States takes its NPT [Nuclear Non-Proliferation Treaty] article VI obligations very seriously. In fact, we reaffirmed them in the Principles and Objectives document of the 1995 Review and Extension Conference of the parties to the NPT. 
United States, Statement before the First Committee of the UN General Assembly, UN Doc. A/C.1/51/PV.22, 14 November 1996, p. 3.
In September 2009, in a speech before the UN General Assembly, the US President stated:
I have outlined a comprehensive agenda to seek the goal of a world without nuclear weapons. In Moscow, the United States and Russia announced that we would pursue substantial reductions in our strategic warheads and launchers. At the Conference on Disarmament, we agreed on a work plan to negotiate an end to the production of fissile materials for nuclear weapons. And this week, my Secretary of State will become the first senior American representative to the annual Members Conference of the Comprehensive Test Ban Treaty.
Today, let me put forward four pillars that I believe are fundamental to the future that we want for our children: non-proliferation and disarmament; the promotion of peace and security; …
First, we must stop the spread of nuclear weapons, and seek the goal of a world without them.
This institution was founded at the dawn of the atomic age, in part because man’s capacity to kill had to be contained. For decades, we averted disaster, even under the shadow of a superpower stand-off. But today, the threat of proliferation is growing in scope and complexity. If we fail to act, we will invite nuclear arms races in every region, and the prospect of wars and acts of terror on a scale that we can hardly imagine.
A fragile consensus stands in the way of this frightening outcome, and that is the basic bargain that shapes the Nuclear Non-Proliferation Treaty. It says that all nations have the right to peaceful nuclear energy; that nations with nuclear weapons have a responsibility to move toward disarmament; and those without them have the responsibility to forsake them. The next 12 months could be pivotal in determining whether this compact will be strengthened or will slowly dissolve.
America intends to keep our end of the bargain. We will pursue a new agreement with Russia to substantially reduce our strategic warheads and launchers. We will move forward with ratification of the Test Ban Treaty, and work with others to bring the treaty into force so that nuclear testing is permanently prohibited. We will complete a Nuclear Posture Review that opens the door to deeper cuts and reduces the role of nuclear weapons. And we will call upon countries to begin negotiations in January on a treaty to end the production of fissile material for weapons. 
United States, Remarks by the President to the UN General Assembly, New York, 23 September 2009.
In December 2009, in his Nobel Peace Prize acceptance speech, the US President stated:
Those regimes that break the rules must be held accountable. Sanctions must exact a real price. Intransigence must be met with increased pressure – and such pressure exists only when the world stands together as one.
One urgent example is the effort to prevent the spread of nuclear weapons, and to seek a world without them. In the middle of the last century, nations agreed to be bound by a treaty whose bargain is clear: All will have access to peaceful nuclear power; those without nuclear weapons will forsake them; and those with nuclear weapons will work towards disarmament. I am committed to upholding this treaty. It is a centerpiece of my foreign policy. 
United States, Remarks by the President at the Acceptance of the Nobel Peace Prize, Oslo, 10 December 2009.
In May 2010 the US President issued the 2010 National Security Strategy, which stated:
Reverse the Spread of Nuclear and Biological Weapons and Secure Nuclear Materials
The American people face no greater or more urgent danger than a terrorist attack with a nuclear weapon. And international peace and security is threatened by proliferation that could lead to a nuclear exchange. Indeed, since the end of the Cold War, the risk of a nuclear attack has increased. Excessive Cold War stockpiles remain. More nations have acquired nuclear weapons. Testing has continued. Black markets trade in nuclear secrets and materials. Terrorists are determined to buy, build, or steal a nuclear weapon. Our efforts to contain these dangers are centered in a global non-proliferation regime that has frayed as more people and nations break the rules.
That is why reversing the spread of nuclear weapons is a top priority. Success depends upon broad consensus and concerted action, we will move forward strategically on a number of fronts through our example, our partnerships, and a reinvigorated international regime. The United States will:
Pursue the Goal of a World Without Nuclear Weapons: While this goal will not be reached during this Administration, its active pursuit and eventual achievement will increase global security, keep our commitment under the NPT [1968 Treaty on the Non-Proliferation of Nuclear Weapons], build our cooperation with Russia and other states, and increase our credibility to hold others accountable for their obligations. As long as any nuclear weapons exist, the United States will sustain a safe, secure, and effective nuclear arsenal, both to deter potential adversaries and to assure U.S. allies and other security partners that they can count on America’s security commitments. But we have signed and seek to ratify a landmark New START Treaty with Russia [New START (Strategic Arms Reduction Treaty) signed on 8 April 2010] to substantially limit our deployed nuclear warheads and strategic delivery vehicles, while assuring a comprehensive monitoring regime. We are reducing the role of nuclear weapons in our national security approach, extending a negative security assurance not to use or threaten to use nuclear weapons against those non-nuclear nations that are in compliance with the NPT and their nuclear non-proliferation obligations, and investing in the modernization of a safe, secure, and effective stockpile without the production of new nuclear weapons. We will pursue ratification of the Comprehensive Test Ban Treaty. And we will seek a new treaty that verifiably ends the production of fissile materials intended for use in nuclear weapons.
Strengthen the Nuclear Non-Proliferation Treaty: The basic bargain of the NPT is sound: countries with nuclear weapons will move toward disarmament; countries without nuclear weapons will forsake them; and all countries can access peaceful nuclear energy. To strengthen the NPT, we will seek more resources and authority for international inspections. … We will pursue a broad, international consensus to insist that all nations meet their obligations. And we will also pursue meaningful consequences for countries that fail to meet their obligations under the NPT or to meet the requirements for withdrawing from it. 
United States, Report by the President, “2010 National Security Strategy”, The White House, Washington DC, 26 May 2010, p. 23; see also pp. 3, 4, 8 and 20.