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Updated January 10, 2020
Defense Primer: Strategic Nuclear Forces
The Nuclear Triad
bombers could be dispersed quickly and launched to
Since the early 1960s the United States has maintained a
enhance their survivability, and they could be recalled to
“triad” of strategic nuclear delivery vehicles. These include
their bases if a crisis did not escalate into conflict.
long-range land-based intercontinental ballistic missiles
(ICBMs), long-range submarine-launched ballistic missiles
The United States has reaffirmed the value of the nuclear
(SLBMs), and long-range heavy bombers equipped to carry
triad. The Obama Administration noted, in the 2010
nuclear-armed cruise missiles and nuclear-armed gravity
Nuclear Posture Review (NPR), that the unique
bombs. The number of nuclear warheads carried on these
characteristics of each leg of the triad were important to the
delivery vehicles peaked in the late 1980s, at around 14,000
goal of maintaining strategic stability at reduced numbers of
warheads. It has been declining ever since, both as the
warheads. It pointed out that strategic nuclear submarines
United States complies with limits in U.S.-Russian arms
(SSBNs) are the most survivable leg of the triad, that
control agreements and as it has changed requirements after
single-warhead ICBMs contribute to stability and are not
the Cold War. At the present time, the United States is
vulnerable to air defenses, and that bombers can be
reducing its forces to comply with the New START Treaty,
deployed as a signal in crisis, to strengthen deterrence and
which entered into force in early 2011. Table 1 displays the
provide assurances to allies and partners. It also noted that
U.S. force structure that is consistent with the treaty limits.
“retaining sufficient force structure in each leg to allow the
Maintenance schedule and operational plans are likely to
ability to hedge effectively by shifting weight from one
reduce the actual number of available warheads.
Triad leg to another if necessary due to unexpected
technological problems or operational vulnerabilities.”
Table 1. U.S. Strategic Nuclear Forces Under New
START
The Trump Administration, in the 2018 Nuclear Posture
Total
Deployed
Warheads
Review, also reaffirmed the U.S. commitment to the
System
Launchers
Launchers
(est.)a
nuclear triad and to the modernization programs for each of
Minuteman
the components of that force structure. It noted that “the
454
400
400
triad’s synergy and overlapping attributes help ensure the
III ICBM
enduring survivability of our deterrence capabilities against
Trident (D-
280
240
1,090
attack and our capacity to hold a range of adversary targets
5) SLBM
at risk throughout a crisis or conflict. Eliminating any leg of
B-52
46
42
42
the triad would greatly ease adversary attack planning and
bombers
allow an adversary to concentrate resources and attention
B-2 bombers
20
18
18
on defeating the remaining two legs.”
Total
800
700
1,550
Current Forces and Modernization Plans
Source: U.S. Department of Defense, Report on Plan to Implement
the Nuclear Force Reductions, Limitations, and Verification,
The United States is currently recapitalizing each leg of its
Washington, DC, April 8, 2014.
nuclear triad and refurbishing many of the warheads carried
by those systems.
a. The treaty attributes only one warhead to each deployed
ICBMs
bomber, although they could carry up to 20 bombs or cruise
missiles.
Until recently, the United States deployed 450 Minuteman
III ICBMs at three Air Force bases: F.E. Warren AFB in
Rationale for the Triad
Wyoming, Malmstrom AFB in Montana, and Minot AFB in
North Dakota. Under the New START Treaty, the number
Early in the Cold War, the United States developed these
has declined to 400 deployed missiles, although the Air
three types of nuclear delivery vehicles, in large part
Force has retained all 450 silo launchers. While each
because each of the military services wanted to play a role
Minuteman III missile originally carried three warheads,
in the U.S. nuclear arsenal. However, during the 1960s and
each now carries a single warhead, both to reduce U.S.
1970s, analysts developed a more reasoned rationale for the
nuclear “triad.” They argued that these different basing
forces to New START levels and to adopt what is
considered a more stabilizing posture.
modes had complementary strengths and weaknesses that
would enhance deterrence and discourage a Soviet first
Over the past 15 years, the Air Force pursued several
strike. For example, ICBMs were believed to have the
programs designed to improve the accuracy and reliability
accuracy and prompt responsiveness needed to attack
of the Minuteman fleet and to, according to the Air Force,
hardened targets such as Soviet command posts and ICBM
“support the operational capability of the Minuteman ICBM
silos, SLBMs had the survivability needed to complicate
through 2030.” In addition, the Air Force is pursuing the
Soviet efforts to launch a disarming first strike and to
Ground-based Strategic Deterrent (GBSD) which will retain
retaliate if such an attack were attempted, and heavy
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Defense Primer: Strategic Nuclear Forces
silo-based ICBMs but replace the entire flight system and
B83 nuclear bombs, but it is not equipped to carry cruise
all the ground launch control facilities. The Air Force plans
missiles. It can also carry conventional weapons and has
to acquire 642 missiles to support testing and the
participated in U.S. military campaigns from Bosnia to Iraq.
deployment of a force of 400 missiles. The Air Force has
The Air Force maintains 76 B-52H bombers at two bases,
estimated that this program will cost $62.3 billion over 30
Barksdale, LA, and Minot, ND. The B-52 bomber, which
years, although the Pentagon’s Cost Assessment and
first entered service in 1961, is equipped to carry nuclear or
Program Evaluation Office has estimated that the cost could
conventional air-launched cruise missiles (ALCMs). The B-
reach $85 billion over 30 years. The Air Force expects the
52 bombers can also deliver a wide range of conventional
program to reach its initial operational capacity, with 9
arms, and are currently receiving numerous upgrades to
missiles on alert, by 2029; it expects to complete the
their communications and electronics systems. The Air
deployment, with 400 missiles on alert, in 2036.
Force is also acquiring a new B-21 bomber, for both
conventional and nuclear missions. It hopes to field
SLBMs
between 80 and 100 of the new bombers, with the first to
The United States currently has 14 Trident (Ohio-class)
enter service around 2025. Costs remain classified.
ballistic missile submarines, with two in overhaul and 12 in
the operational fleet. Under the New START Treaty, each
According to unclassified estimates, the United States has
of the submarines carries only 20, rather than the original
around 475 B61 and B83 bombs. Several variants of the
24, missiles. Using treaty counting rules, the 14 submarines
B61 bomb are undergoing a life extension program (LEP),
count as a total of 280 deployed and nondeployed
to enhance the bomb’s safety, security, and use control
launchers, with 240 deployed launchers and around 1,000
features. The Air Force is also designing a new tail kit for
warheads counting on the 12 operational submarines. The
the B61 bomb, replacing the parachute currently used to
Navy operates two bases for these submarines—one in
slow the bomb’s descent, to improve its accuracy. NNSA
Bangor, WA, and one in Kings Bay, GA.
estimates that the B61 LEP will cost nearly $9 billion, with
an additional $1-$2 billion in Air Force funding for the new
The Navy plans to begin retiring Ohio-class submarines
tailkit. The new model was expected to begin to enter the
from the fleet in 2027; it is designing a new Columbia-class
force in 2020, but concerns with some parts used in the LEP
submarine that is expected to begin entering the fleet in
may delay that date. NNSA had planned to retire the B83,
2031. The Navy initially estimated that each new submarine
the largest bomb remaining in the U.S. arsenal, around
could cost $6 billion to $7 billion in FY2010 dollars, but it
2025, after the completion of the B61 LEP, but, according
has worked to redesign the submarine and reduce the costs,
to the 2018 NPR, may retain it for a longer time.
with the plan to hold each submarine to around $4.9 billion,
in FY2010 dollars. The Navy has recently indicated that,
The Air Force is planning to replace the aging air-launched
using then-year dollars rather than 2010 dollars, the fleet of
cruise missiles carried by B-52 bombers with a new
12 new submarines will cost $139 billion.
advanced long range standoff (LRSO) cruise missile.
According to the Air Force, the existing ALCM has been
The Navy purchased over 530 D-5 missiles to support the
through several life extension programs and is beginning to
Trident fleet. It is now pursuing a life-extension program
show reliability problems. Reports indicate that the Air
for the missiles, so that they will remain capable and
Force plans to buy a total of 1,000-1,100 LRSO missiles, at
reliable throughout the life of the Ohio-class submarines
a cost of around $10.8 billion, to support the testing
and into the deployment of a new, Columbia-class
program and deployment plans over the life of the missile.
submarine. The Navy currently spends approximately $1
NNSA is also conducting a life-extension program on the
billion per year on this life extension program.
W80 warhead to provide a warhead for the new LRSO.
The National Nuclear Security Administration (NNSA), a
CRS Products
semi-autonomous agency in the Department of Energy, is
also working to extend the life of the warheads carried by
CRS Report R41219, The New START Treaty: Central Limits and
U.S. SLBMs. NNSA completed a life extension program
Key Provisions, by Amy F. Woolf
(LEP) for the W76 warhead, which is carried by most
CRS Report RL33640, U.S. Strategic Nuclear Forces: Background,
Trident missiles, in 2019. This LEP is intended to add 30
Developments, and Issues, by Amy F. Woolf
years to the warhead life “by refurbishing the nuclear
explosive package, the arming, firing, and fusing system,
the gas transfer system, and many other associated
components.” The LEP also provided a small number of
Other Resources
low-yield warheads, known as the W76-2, as mandated by
the 2018 NPR. NNSA is also altering the W88 warhead,
DOD. Nuclear Posture Review Report. April 2010.
which is carried by a portion of the fleet, to address
State Department. New START Treaty. April 2010
concerns with its safety and reliability. In particular, the
program will replace the aging arming, fuzing, and firing
components.
Amy F. Woolf, Specialist in Nuclear Weapons Policy
Heavy Bombers
The Air Force has 20 B-2 bombers, based at Whiteman
IF10519
AFB in Missouri. The B-2 bomber can carry both B61 and
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Defense Primer: Strategic Nuclear Forces
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