A no-first-use nuclear policy means that the United States would commit to never being the first nation to use nuclear weapons in any conflict, a change from its current policy.
Under current policy, the United States will not use nuclear weapons against the vast majority of the world's countries in any circumstances. Longstanding US policy, re-affirmed in the Trump administration's 2018 Nuclear Posture Review (NPR), says that the United States "will not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT [Nuclear Nonproliferation Treaty] and in compliance with their nuclear nonproliferation obligations"; this promise covers more than 180 countries (OSD 2018). This policy is known as a "negative security assurance."
However, China, Russia, and North Korea do not fall under the US negative security assurance. China and Russia are nuclear weapon states under the NPT, and North Korea withdrew from the treaty in 2003 and conducted its first nuclear test in 2006.
This means that they could be targets for US nuclear weapons, including the United States launching weapons at them first.
One noteworthy thing about today’s landscape is that the Trump administration's NPR has significantly expanded the definition of "extreme." Both the Obama and Trump administration NPRs state that the United States “would only consider the use of nuclear weapons in extreme circumstances to defend the vital interests of the United States or its allies and partners.” However, the Obama version stated that use would be limited to “a narrow range of contingencies” and emphasized that the goal was to continue to “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 our allies and partners the sole purpose of US nuclear weapons.”
The Trump NPR broadens the definition of “extreme circumstances,” saying these “could include significant non-nuclear attacks. Significant non-nuclear attacks include, but are not limited to, attacks on the U.S., allies, or partner civilian population or infrastructure, and attacks on U.S. or allied nuclear forces, their command and control, or warning and attack assessment.” This could potentially include cyber attacks as a valid reason for nuclear use under US policy.
A commitment to never using nuclear weapons first
A no-first-use nuclear policy means that the United States would commit to never use nuclear weapons first, either as a first strike (that is, an unprompted surprise attack), as an escalatory move in a conventional conflict, or in response to a non-nuclear attack.
The only situation in which the US would use nuclear weapons would be in response to a confirmed nuclear attack on itself or its allies.
What’s the difference between a "no-first-use" declaration and a "sole purpose" declaration?
No-first-use and sole purpose are essentially the same. In a sole-purpose declaration, a country states that the only purpose of its nuclear weapons is to deter or respond to a nuclear attack by another country. Such a pledge would rule out using nuclear weapons first.
How would adopting NFU make the US safer?
There are many reasons why retaining the option of using nuclear weapons first is dangerous for the United States. If the president decided to cross the threshold and use nuclear weapons first against a nuclear-armed adversary (Russia, China, or North Korea), those countries would almost certainly retaliate with nuclear weapons, either directly against the US or against its allies. Maintaining a first-use option therefore increases the chance of a catastrophic attack against the US public.
In addition, if a nuclear-armed adversary is concerned that the United States might use nuclear weapons first in a crisis, that could increase the adversary's incentive to go nuclear first because of "use-it-or-lose-it" thinking—that is, the fear that if the US attacked first it might wipe out the adversary's nuclear arsenal. This kind of thinking creates pressure to use these weapons before they are lost.
Taking nuclear use off the table except as a retaliatory measure could reduce this pressure, which would help to slow the timeline in a crisis, allowing decision-makers more time to explore other solutions rather than quickly escalating the conflict.
An NFU policy would also reduce concerns about the US president's sole authority to order a nuclear attack, since these concerns have focused primarily on a possible order to initiate the use of nuclear weapons in a crisis. An NFU policy would remove the option for the president to order the use of nuclear weapons except in retaliation for a nuclear attack.
It’s also worth keeping in mind that the United States already has a no-first-use policy that applies to the vast majority of the world's countries. The rationale for that pledge is to reassure countries without nuclear weapons that they do not need nuclear weapons to deter a nuclear attack against them. This benefits US security by minimizing the number of nuclear-armed potential adversaries it has to deal with.
The United States apparently thinks that most of the world's countries believe this pledge, and thinks it is safe to make such a NFU pledge with respect to these countries.
Do any other countries have no-first-use policies?
China has had an unconditional no-first-use policy since it first developed nuclear weapons in 1964, and has repeatedly reaffirmed that position, most recently in a Ministry of Defense white paper on national defense published in July 2019. Although some critics say this policy has little value because it is merely declarative and could change, China fields its nuclear forces in a way that is consistent with an NFU policy. Unlike the US and Russia, China stores its nuclear warheads separately from its missiles. It would take time to mate the warheads to missiles before China could launch an attack, making it unlikely that a Chinese nuclear attack would be a surprise.
Russia had a no-first-use policy from 1982 until 1993, when it changed its policy out of fear that its weakened conventional forces could no longer deter the United States without the threat of use of nuclear weapons. In 2003, India adopted a conditional no-first-use policy that reserves the right to use nuclear weapons if Indian forces are attacked with biological or chemical weapons.