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Updated December 3, 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. As of February 2018, the United States had
vulnerable to air defenses, and that bombers can be
reduced 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. forces that count under the treaty limits, as of
“retaining sufficient force structure in each leg to allow the
September 1, 2020. These forces fall below the treaty limits
ability to hedge effectively by shifting weight from one
of 1,550 deployed warheads on 700 deployed missiles due
Triad leg to another if necessary due to unexpected
to maintenance schedules and operational requirements.
technological problems or operational vulnerabilities.”
Table 1. U.S. Strategic Nuclear Forces Under New
The Trump Administration, in the 2018 Nuclear Posture
START
Review, also reaffirmed the U.S. commitment to the
Total
Deployed
nuclear triad and to the modernization programs for each of
System
Launchers
Launchers
Warheads
the components of that force structure. It noted that “the
triad’s synergy and overlapping attributes help ensure the
Minuteman
454
397
397
enduring survivability of our deterrence capabilities against
III ICBM
attack and our capacity to hold a range of adversary targets
Trident (D-
280
230
1,012
at risk throughout a crisis or conflict. Eliminating any leg of
5) SLBM
the triad would greatly ease adversary attack planning and
B-52
46
36
36a
allow an adversary to concentrate resources and attention
bombers
on defeating the remaining two legs.”
B-2 bombers
20
12
12a
Total
800
675
1,467
Current Forces and Modernization Plans
Source: U.S. Department of State. New START Treaty Aggregate
The United States is currently recapitalizing each leg of its
Numbers of Strategic Offensive Arms, December 1, 2020.
nuclear triad and refurbishing many of the warheads carried
by those systems.
a. The treaty attributes only one warhead to each deployed
bomber, although they could carry up to 20 bombs or cruise
ICBMs
missiles.
Before implementing the New START Treaty, the United
States deployed 450 Minuteman III ICBMs at three Air
Rationale for the Triad
Force bases: F.E. Warren AFB in Wyoming, Malmstrom
Early in the Cold War, the United States developed these
AFB in Montana, and Minot AFB in North Dakota. Under
three types of nuclear delivery vehicles, in large part
New START, the number has declined to 400 deployed
because each of the military services wanted to play a role
missiles, although the Air Force has retained all 450 silo
in the U.S. nuclear arsenal. However, during the 1960s and
launchers. While each Minuteman III missile originally
1970s, analysts developed a more reasoned rationale for the
carried three warheads, each now carries a single warhead,
nuclear “triad.” They argued that these different basing
both to reduce U.S. forces to New START levels and to
modes had complementary strengths and weaknesses that
adopt what is considered a more stabilizing posture.
would enhance deterrence and discourage a Soviet first
strike. For example, ICBMs were believed to have the
Over the past 20 years, the Air Force pursued several
accuracy and prompt responsiveness needed to attack
programs designed to improve the accuracy and reliability
hardened targets such as Soviet command posts and ICBM
of the Minuteman fleet and to, according to the Air Force,
silos, SLBMs had the survivability needed to complicate
“support the operational capability of the Minuteman ICBM
Soviet efforts to launch a disarming first strike and to
through 2030.” In addition, the Air Force is developing the
retaliate if such an attack were attempted, and heavy
Ground-based Strategic Deterrent (GBSD) which will retain
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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 had estimated that the cost
conventional air-launched cruise missiles (ALCMs). The B-
could reach $85 billion over 30 years. The Air Force
52 bombers can also deliver a wide range of conventional
expects the program to reach its initial operational capacity,
arms, and are currently receiving numerous upgrades to
with 9 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
The National Nuclear Security Administration (NNSA), a
between 80 and 100 of the new bombers, with the first to
semi-autonomous agency in the Department of Energy, is
enter service around 2025. Costs remain classified.
beginning work on a new warhead—known as the W87-1—
that will deploy on the new GBSD missile.
According to unclassified estimates, the United States has
around 475 B61 and B83 bombs. Several variants of the
SLBMs
B61 bomb are undergoing a life extension program (LEP),
The United States currently has 14 Trident (Ohio-class)
to enhance the bomb’s safety, security, and use control
ballistic missile submarines, with two in overhaul and 12 in
features. This program will produce a single variant, known
the operational fleet. Under the New START Treaty, each
as the B61-12, which will deploy with a new tail kit. This
of the submarines carries only 20, rather than the original
will replace the parachute currently used to slow the
24, missiles. Using treaty counting rules, the 14 submarines
bomb’s descent and will improve its accuracy. NNSA
count as a total of 280 deployed and nondeployed
estimates that the B61 LEP will cost nearly $9 billion, with
launchers, with a maximum of 240 deployed launchers and
an additional $1-$2 billion in Air Force funding for the new
around 1,000 warheads counting on the 12 operational
tailkit. The new model was expected to begin to enter the
submarines. The Navy operates two bases for these
force in 2020, but concerns with some parts used in the LEP
submarines—one in Bangor, WA, and one in Kings Bay,
may delay that date. NNSA had planned to retire the B83,
GA.
the largest bomb remaining in the U.S. arsenal, around
2025, after the completion of the B61 LEP, but, according
The Navy plans to begin retiring Ohio-class submarines
to the 2018 NPR, may retain it for a longer time.
from the fleet in 2027; it is beginning the production of a
new Columbia-class submarine that is expected to begin
The Air Force is planning to replace the aging air-launched
entering the fleet in 2031. The Navy initially estimated that
cruise missiles carried by B-52 bombers with a new
each new submarine could cost $6 billion to $7 billion in
advanced long range standoff (LRSO) cruise missile.
FY2010 dollars, but it has worked to redesign the
According to the Air Force, the existing ALCM has been
submarine and reduce the costs. The Navy has recently
through several life extension programs and is beginning to
indicated that the fleet of 12 new submarines will cost $139
show reliability problems. Reports indicate that the Air
billion.
Force plans to buy a total of 1,000-1,100 LRSO missiles, at
a cost of around $10.8 billion, to support the testing
The Navy purchased over 530 D-5 missiles to support the
program and deployment plans over the life of the missile.
Trident fleet. It is now pursuing a life-extension program
NNSA is also conducting a life-extension program on the
for the missiles, so that they will remain capable and
W80 warhead to provide a warhead for the new LRSO.
reliable throughout the life of the Ohio-class submarines
and into the deployment of a new, Columbia-class
CRS Products
submarine. The Navy currently spends approximately $1
billion per year on this life extension program.
CRS Report R41219, The New START Treaty: Central Limits and
Key Provisions, by Amy F. Woolf
NNSA is also working to extend the life of the warheads
CRS Report RL33640, U.S. Strategic Nuclear Forces: Background,
carried by U.S. SLBMs. It has conducted a life extension
Developments, and Issues, by Amy F. Woolf
program (LEP) for the W76 warhead, which is carried by
most Trident missiles, and provided a small number of low-
yield warheads, known as the W76-2, to the Navy in late
2019. NNSA is also altering the W88 warhead, which is
Other Resources
carried by a portion of the fleet, to address concerns with its
safety and reliability. In particular, the program will replace
DOD. Nuclear Posture Review Report. April 2010.
the aging arming, fuzing, and firing components. It has
State Department. New START Treaty. April 2010
requested funding to begin a study on the W93 warhead,
which will eventually deploy on D-5 missiles.
Heavy Bombers
Amy F. Woolf, Specialist in Nuclear Weapons Policy
The Air Force has 20 B-2 bombers, based at Whiteman
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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