Updated January 5, 2021
A Low-Yield, Submarine-Launched Nuclear Warhead:
Overview of the Expert Debate
The Low-Yield D-5 Warhead
argue that adversaries might mistakenly believe the United
The Trump Administration developed a new low-yield
States would be self-deterred from responding with nuclear
version of the W-76 warhead for existing submarine-
weapons after an adversary’s nuclear use in a regional
launched Trident II (D-5) missiles. Unclassified sources
conflict, and therefore could be coerced into withdrawing
state that the existing W76-1 warhead has an explosive
from the fight if an adversary threatened nuclear use. They
yield of around 100 kilotons. The National Nuclear Security
contend that Russia in particular might threaten to escalate
Administration (NNSA) has said the low-yield version, the
to nuclear weapons if it were losing a conventional conflict,
W76-2, would be configured “for primary-only detonation.”
and note that Russia has exercised the use of low-yield
This could mean a yield of less than 10 kilotons.
nuclear weapons for this type of contingency. They argue
that if Russia pursued this approach, the United States
Congress appropriated $65 million for the W76-2 warhead
would only be able to respond with the higher-yield
in FY2019 and $10 million to complete work in FY2020. It
weapons like those currently deployed on submarine-
also authorized $19.6 million in FY2020 for the Navy to
launched missiles. The deployment of a low-yield D-5
integrate the warhead into the submarine force. NNSA
warhead would therefore bolster deterrence by convincing
completed the first modified warhead in February 2019,
Russia that the United States could respond with a
began delivering warheads to the Navy by late 2019, and
proportional, limited attack.
completed the deliveries during FY2020. The Pentagon
reported in February 2020 that the Navy had begun
Critics of the NPR’s analysis question whether the United
deploying the warheads by that time. NNSA did not
States needs a new weapon to address Russia’s mistaken
disclose the total number produced, although it is likely just
belief that it could threaten escalation without fearing U.S.
a very small portion of the W76 stockpile (estimated, in
retaliation. If the belief is mistaken, they argue, then the
unclassified sources, to be around 1,300 total warheads).
United States could respond by reasserting and reaffirming
its commitment to its allies in Europe, so that Russia would
The Trump Administration introduced the low-yield version
know that this type of threat would not be met with a U.S.
of the W76 warhead in the 2018 Nuclear Posture Review
or NATO retreat. They also contend that the deployment of
(NPR). It cited the need for additional “tailored” and
new low-yield options could increase the risk of nuclear
“flexible” capabilities to address the danger of coercive
war because their existence would make it easier for U.S.
nuclear use, a concept described below, by Russia and
officials to consider the use of nuclear weapons in a
North Korea. The NPR stated that this warhead would
conflict. Some have also argued that there is no “gap” in
supplement existing U.S. strategic nuclear capabilities to
capabilities because the United States already has low-yield
“enhance deterrence by denying potential adversaries any
warhead options for gravity bombs and cruise missiles
mistaken confidence that limited nuclear employment can
deployed on U.S. and NATO aircraft.
provide a useful advantage over the United States and its
allies,” and that low-yield warheads would not add to the
On these latter points, those who support the NPR’s
number of deployed SLBM warheads, but would replace
analysis have pointed out that the low-yield SLBM could
some “higher-yield [SLBM warheads] currently deployed.”
improve survivability and penetration as weapons delivered
by aircraft would be vulnerable to an adversary’s air
The NPR report, and its argument in favor of a low-yield
defenses. Some also cite the U.S. experience of deploying
SLBM warhead, launched a debate among U.S. experts
lower-yield nuclear weapons during the Cold War to posit
about the rationale for the development of such a warhead
that there is no evidence that the United States is more
and the benefits and risks that might accrue from its
likely to use these weapons just because it has them.
deployment. While some argue that this warhead is a
response to Russia’s so-called “escalate to de-escalate”
The Potential for Limited Nuclear War
strategy that will strengthen deterrence and raise the nuclear
The debate has also included discussions about whether a
threshold, others contend that it will lower the threshold for
war in which nations used small numbers of low-yield
U.S. use and increase the risk of nuclear war.
nuclear weapons could remain “limited,” or whether it
would inevitably escalate to a more extensive nuclear
Deterrence vs. Warfighting
exchange. The NPR’s analysis rests on the view that
The core of the debate over the low-yield D-5 warhead
Russian might use a limited number of nuclear weapons if it
focuses on the question of whether the United States has a
is losing a conventional war, and that the United States
gap in its current nuclear deterrent capabilities that can be
should be able to threaten a limited response to deter
filled by the deployment of a new low-yield warhead. The
Russia. Critics have countered that there is no such thing as
2018 NPR and experts who support the report’s assessment
“limited” nuclear war because any use of a nuclear weapon
A Low-Yield, Submarine-Launched Nuclear Warhead: Overview of the Expert Debate
would make a conflict something more than limited. Even if
daunting search area, making it very difficult for Russia to
the numbers are small and the yields are low, they argue,
pinpoint the boat’s location with enough confidence to
the damage would be extensive. They have also argued that
launch a successful attack.
nuclear war could not be controlled, so even the limited use
of nuclear weapons would risk a global catastrophe.
Considerations about a potential reduction in collateral
Some analysts dispute the idea that nuclear war cannot
damage have also entered into the debate about the
remain limited. Others, however, agree that the use of
development of low-yield SLBM warheads. The U.S.
nuclear weapons would increase the risk of broader
military has generally favored, based on the Law of Armed
escalation and see this as a point in favor of the U.S.
Conflict, providing the President with nuclear options that
deployment of low-yield nuclear weapons. They argue that
have “less collateral effect.” By extension, some experts
Russia seems to believe that it could use nuclear weapons in
have posited the need for a “nuclear necessity principle,”
a limited way and deter the United States from responding
where U.S. nuclear planners would “use the lowest-yield
with its larger warheads. By deploying a low-yield SLBM
nuclear weapon possible,” and only in cases where
warhead, the United States would not only aim to convince
hardened and buried targets could not be destroyed by
Russia that the United States would respond after a limited
conventional weapons. A low-yield D-5 warhead, they
attack, but would also bolster deterrence precisely because
argue, would support this goal.
Russia’s limited use of nuclear weapons could lead to an
Others counter that the lower-yield warhead and less-
escalation to a broader nuclear exchange.
stringent use parameters would actually increase the risk of
nuclear use in a conflict. This, they argue, would actually
In disputing this analysis, some have questioned the NPR’s
increase the risk of nuclear war, and therefore increase the
assessment of Russian nuclear doctrine and have countered
risk of devastating nuclear destruction, possibly in violation
that the NPR’s assertion that Russia has lowered its nuclear
of the Law of Armed Conflict.
threshold is not based on sufficient evidence. They argue
that the possible first use of nuclear weapons by Russia and
North Korea would likely have less to do with a coercive
Anya Fink provided valued assistance in preparing this report.
nuclear strategy intended to deter the United States than
with these countries’ concerns about U.S. conventional
superiority—that they would resort to nuclear weapons
because they could not fight and win a conventional war.
The Discrimination Problem
CRS Report RL33640, U.S. Strategic Nuclear Forces: Background,
Some experts have posited that the deployment of a low-
Developments, and Issues
, by Amy F. Woolf
yield SLBM warhead could create a new “discrimination
problem,” in which an adversary like Russia would be
unable to distinguish during a conflict if an SLBM launched
by the United States carried just one low-yield warhead and
was not part of a large attack. In this view, a U.S. launch
intended to control the escalation of a regional conflict
U.S. Department of Defense, Nuclear Posture Review
could contribute to Russia’s decision to escalate to the
Washington, DC, February 5, 2018.
strategic level due to misinterpretation and doubts about its
John R. Harvey, Franklin C. Mil er, Keith B. Payne, and Bradley
early warning systems’ accuracy.
H. Roberts, “Continuity and Change in U.S. Nuclear Policy,”
Others have disputed this assessment, arguing that the U.S.
, February 7, 2018.
policy of “limited nuclear options” has historically been,
Jon Wolfsthal, “Say No to New, Smaller Nuclear Weapons,”
and continues to be, based on assessments that Russia’s
War on the Rocks
, November 22, 2017.
early warning systems could tell the difference between a
Francis J. Gavin, chair, “Policy Roundtable: The Trump
single launch and large attack. They contend that Russia
Administration’s Nuclear Posture Review,” Texas National
would likely delay its response until it had made that
, February 13, 2018.
assessment. They also claim that the novelty of this
“discrimination problem” is overstated because the United
Daryl Kimball and Kingston Reif, “The New U.S. Nuclear
Strategy is Flawed and Dangerous. Here’s Why,” Arms
Kingdom already deploys low-yield warheads on its
Control Association, February 15, 2018.
SLBMs, and the United States and United Kingdom rely on
a “common pool” of Trident II D5 missiles—yet no one has
Austin Long, “Discrimination Details Matter,” War on the
ever claimed that this arrangement might lead to confusion
, February 16, 2018, also Austin Long, “Location,
about the size or scale of a U.S. retaliatory attack.
Location, Location,” Lawfare, March 11, 2018.
Scott Sagan, “Armed and Dangerous,” Foreign Affairs
Some have advanced the argument that U.S. ballistic
missile submarines could be vulnerable to detection after
the launch of a single or small number of missiles carrying
low-yield warheads because the launch would reveal the
Amy F. Woolf
, Specialist in Nuclear Weapons Policy
boat’s location. Others have countered that the boat would
be able to move quickly enough to create a large, possibly
A Low-Yield, Submarine-Launched Nuclear Warhead: Overview of the Expert Debate
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