An inert Minuteman 3 missile sits in a training launch tube at Minot Air Force Base in North Dakota on June 25, 2014. The base is tasked with maintaining 150 nuclear-tipped missiles and keeping them ready to launch at a moment's notice. (Charlie Riedel/AP)

Tuesday’s debate among Democratic presidential candidates focused largely on domestic policy. But national security briefly entered the fray when moderators asked Sen. Elizabeth Warren (Mass.) about her support for a nuclear weapons “No First Use” (NFU) policy. Warren later tweeted that “threatening to use nuclear weapons first makes America less safe.”

Democratic support for NFU is growing. As she noted in her tweet, Warren has introduced a bill making NFU U.S. policy. Rep. Adam Smith (D-Wash.), chairman of the Armed Services Committee, has introduced Warren’s legislation in the House. Nearly 40 House and Senate Democrats — and no Republicans — have signed on.

How does NFU differ from current U.S. policy, and what are the purported benefits? Here are five things to know:

1. What does NFU mean?

NFU would change nuclear “declaratory policy” — what the United States says publicly about its nuclear strategy. Under an NFU policy, the United States would declare that it would not use nuclear weapons unless it or its allies were attacked with nuclear weapons. Such a declaration could appear in a future Nuclear Posture Review — the Pentagon-led nuclear policy review typically undertaken by new administrations.

2. What is current policy?

The United States does not have a nuclear first-strike policy.

Rather, the United States maintains a number of nuclear war plans tailored to a variety of adversaries and contingencies. The rationale behind multiple options is to give presidents maximum flexibility so that they have the option of either striking first or absorbing a strike and retaliating.

Analysts believe — the details are secret — that U.S. war plans involve some first-strike options. But declassified Reagan-era documents reveal that U.S. war plans were premised on the assumption that U.S. nuclear use would take place after a Soviet attack began. Current war plans probably also assume that U.S. nuclear first use is the exception rather than the rule.

The most vexing challenge for NFU is whether to retain the ability to preemptively destroy an adversary’s nuclear forces if an attack appears imminent. These “damage limitation” attacks have been an enduring feature of U.S. nuclear war plans. NFU proponents are not clear on whether the policy applies to preemptive strikes when attacks appear imminent.

Warren’s NFU proposal would not change the president’s sole authority to use nuclear weapons. And there would be no effect on U.S. nuclear capabilities, including an active stockpile of about 2,200 nuclear weapons. Of these, 850 warheads on 500 missiles are ready to fire at a moment’s notice — the rest can be loaded onto aircraft relatively quickly. Only a handful of NFU proponents have called for corresponding changes to U.S. nuclear capabilities.

3. What is NFU supposed to accomplish?

Current policy states that the United States would consider nuclear use only in “extreme circumstances.” The definition of “extreme” is intentionally ambiguous and could entail first or second use. The 2018 Nuclear Posture Review states that “extreme” could include (but isn’t limited to) attacks on U.S. or allied civilian populations or infrastructure, or attacks on U.S. or allied nuclear forces and infrastructure.

Proponents of NFU argue that this ambiguity is dangerous. Because the current formulation keeps U.S. nuclear “red lines” fuzzy, adversaries in a future war could not be sure which military or nonmilitary actions might trigger U.S. nuclear use. They might, therefore, opt to jump the gun and launch their own first strike. Consequently, NFU could reduce the risk of miscalculation.

Critics argue that the ambiguity is a feature rather than a bug: If adversaries do not know what might trigger a first strike, they might be deterred from fighting in the first place.

4. Has NFU been tried before?

India, China and the Soviet Union have issued NFU pledges. India’s policy includes a caveat that a biological or chemical attack could warrant nuclear retaliation. China’s NFU policy is unqualified, although analysts suggest that exceptions exist.

The Soviet Union issued a no-first-use pledge in 1982, but Russia rescinded it after the U.S.S.R. collapsed. President Barack Obama considered NFU in his second term but reportedly demurred.

5. How might U.S. adversaries and allies react to NFU?

Critics argue that NFU would undermine the United States’ ability to defend its allies. Allies could also see NFU as a sign of weakening U.S. commitment and, consequently, pursue their own nuclear weapons. Allies might also demand more military aid or even resort to appeasing adversaries. Proponents argue that U.S. conventional military superiority is sufficient to defend U.S. allies and territory.

With respect to U.S. allies, the case for NFU rests on two assumptions: that the United States can remain militarily dominant and that the threat of first use makes no contribution to allied security. If U.S. superiority is in question, or if nuclear weapons make even a marginal contribution to deterrence, NFU might be risky.

Another possibility is that NFU neither helps nor hurts. Here’s an example: The United States found the Soviet Union’s NFU pledge somewhat credible, but Reagan administration officials dismissed it as “entirely self-serving.” The U.S.S.R. had a huge conventional military advantage in Europe, which made it less likely to use nuclear weapons compared with the relative weaker NATO (although the officials stressed that the United States would never initiate conflict in Europe).

For different reasons, Cold War intelligence estimates also found China’s NFU pledge credible. China had a small and unsophisticated nuclear arsenal — so U.S. intelligence analysts concluded that NFU was “probably a realistic statement of intent.” In both cases, NFU pledges were seen as credible but not very significant. The Soviets had no need for first use, while the Chinese lacked the ability to engage in first use.

Even if the U.S. government adopts NFU, either through presidential order or legislation, there are still a host of nuclear challenges, including an ongoing U.S. modernization program and an accelerating arms race in conventional missiles.

The next president will also face the broader challenge of how to sustain U.S. military strength into the future. All of these choices will affect the expected costs and benefits of an NFU policy.

Brian Radzinsky is a PhD candidate-in-residence at George Washington University’s Institute for Security and Conflict Studies, where he is completing a dissertation on the evolution of U.S. and Russian nuclear command and control. Twitter: @b_radzinsky.