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Updated November 12, 2021
Defense Primer: Strategic Nuclear Forces
The Nuclear Triad
Soviet efforts to launch a disarming first strike and to
Since the early 1960s the United States has maintained a
retaliate if such an attack were attempted, and heavy
“triad” of strategic nuclear delivery vehicles. These include
bombers could be dispersed quickly and launched to
long-range land-based intercontinental ballistic missiles
enhance their survivability, and they could be recalled to
(ICBMs), long-range submarine-launched ballistic missiles
their bases if a crisis did not escalate into conflict.
(SLBMs), and long-range heavy bombers equipped to carry
nuclear-armed cruise missiles and nuclear-armed gravity
The United States has reaffirmed the value of the nuclear
bombs. The number of nuclear warheads carried on these
triad. The Obama Administration noted, in the 2010
delivery vehicles peaked in the late 1980s, at around 14,000
Nuclear Posture Review (NPR), that the unique
warheads. It has been declining ever since, both as the
characteristics of each leg of the triad were important to the
United States complies with limits in U.S.-Russian arms
goal of maintaining strategic stability at reduced numbers of
control agreements and as it has changed requirements after
warheads. It pointed out that strategic nuclear submarines
the Cold War. As of February 2018, the United States had
(SSBNs) are the most survivable leg of the triad, that
reduced its forces to comply with the New START Treaty,
single-warhead ICBMs contribute to stability and are not
which entered into force in early 2011. Table 1 displays the
vulnerable to air defenses, and that bombers can be
U.S. forces that count under the treaty limits, as of
deployed as a signal in crisis, to strengthen deterrence and
September 1, 2020. These forces fall below the treaty limits
provide assurances to allies and partners. It also noted that
of 1,550 deployed warheads on 700 deployed missiles due
“retaining sufficient force structure in each leg to allow the
to maintenance schedules and operational requirements. An
ability to hedge effectively by shifting weight from one
update released in March 2021 indicated that the United
Triad leg to another if necessary due to unexpected
States had 1,357 warheads deployed on 651 missiles and
technological problems or operational vulnerabilities.”
bombers.
The Trump Administration also reaffirmed the U.S.
Table 1. U.S. Strategic Nuclear Forces Under New
commitment to the nuclear triad and to the modernization
START
programs for each of the components of that force structure.
Total
Deployed
It noted in the 2018 NPR that “the triad’s synergy and
System
Launchers
Launchers
Warheads
overlapping attributes help ensure the enduring
Minuteman
survivability of our deterrence capabilities against attack
454
397
397
and our capacity to hold a range of adversary targets at risk
III ICBM
throughout a crisis or conflict. Eliminating any leg of the
Trident
280
230
1,012
triad would greatly ease adversary attack planning and
(D-5) SLBM
allow an adversary to concentrate resources and attention
B-52
46
36
36
on defeating the remaining two legs.”
a
The Biden
bombers
Administration will likely review the rationale for a triad
B-2 bombers
20
12
12a
again in its ongoing NPR.
Total
800
675
1,467
Current Forces and Modernization Plans
Source: U.S. Department of State. New START Treaty Aggregate
Numbers of Strategic Offensive Arms, December 1, 2020.
The United States is recapitalizing each leg of its nuclear
triad and refurbishing many of the warheads carried by
a. The treaty attributes one warhead to each deployed bomber,
those systems.
although they could carry up to 20 bombs or cruise missiles.
ICBMs
Rationale for the Triad
Before implementing the New START Treaty, the United
Early in the Cold War, the United States developed these
States deployed 450 Minuteman III ICBMs at three Air
three types of nuclear delivery vehicles, in large part
Force bases: F.E. Warren AFB in Wyoming, Malmstrom
because each of the military services wanted to play a role
AFB in Montana, and Minot AFB in North Dakota. Under
in the U.S. nuclear arsenal. However, during the 1960s and
New START, the number has declined to 400 deployed
1970s, analysts developed a more reasoned rationale for the
missiles, although the Air Force has retained all 450 silo
nuclear “triad.” They argued that these different basing
launchers. While each Minuteman III missile originally
modes had complementary strengths and weaknesses that
carried three warheads, each now carries a single warhead,
would enhance deterrence and discourage a Soviet first
both to reduce U.S. forces to New START levels and to
strike. For example, ICBMs were believed to have the
adopt what is considered a more stabilizing posture.
accuracy and prompt responsiveness needed to attack
hardened targets such as Soviet command posts and ICBM
The Air Force has completed several programs designed to
silos, SLBMs had the survivability needed to complicate
improve the accuracy and reliability of the Minuteman fleet
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Defense Primer: Strategic Nuclear Forces
and to “support the operational capability ... through 2030.”
B83 nuclear bombs, but it is not equipped to carry cruise
The Air Force is also developing a new ICBM, the Ground-
missiles. It can also carry conventional weapons and has
based Strategic Deterrent (GBSD), which will replace all
participated in U.S. military campaigns from Bosnia to Iraq.
missiles and ground launch control facilities. It plans to
The Air Force maintains 76 B-52H bombers at two bases,
acquire 642 missiles to support testing and the deployment
Barksdale, LA, and Minot, ND. The B-52 bomber, which
of a force of 400 missiles. The Air Force has estimated that
first entered service in 1961, is equipped to carry nuclear or
this program will cost $62.3 billion over 30 years, although
conventional air-launched cruise missiles (ALCMs). The B-
the Pentagon’s Cost Assessment and Program Evaluation
52 bombers can also deliver a wide range of conventional
Office had estimated that the cost could reach $85 billion
arms, and are currently receiving numerous upgrades to
over 30 years. The Air Force expects the program to reach
their communications and electronics systems. The Air
its initial operational capacity, with 9 missiles on alert, by
Force is also acquiring a new B-21 bomber, for both
2029; it expects to complete the deployment, with 400
conventional and nuclear missions. It hopes to field
missiles on alert, in 2036.
between 80 and 100 of the new bombers, with the first to
enter service around 2025. Costs remain classified.
The National Nuclear Security Administration (NNSA), a
semi-autonomous agency in the Department of Energy, is
According to unclassified estimates, the United States has
working on a new warhead—known as the W87-1—that
around 475 B61 and B83 bombs. Several variants of the
will deploy on the new GBSD missile.
B61 bomb are undergoing a life extension program (LEP),
to enhance the bomb’s safety, security, and use control
SLBMs
features. This program will produce a single variant, known
The United States currently has 14 Trident (Ohio-class)
as the B61-12, which will deploy with a new tail kit. This
ballistic missile submarines, with 2 in overhaul and 12 in
will replace the parachute currently used to slow the
the operational fleet. Under New START, each submarine
bomb’s descent and will improve its accuracy. NNSA
carries only 20, rather than the original 24, missiles. Using
estimates that the B61 LEP will cost nearly $9 billion, with
treaty counting rules, the 14 submarines count as a total of
an additional $1-$2 billion in Air Force funding for the new
280 deployed and nondeployed launchers, with a maximum
tailkit. The new model was expected to begin to enter the
of 240 deployed launchers and around 1,000 warheads
force in 2020, but concerns with some parts used in the LEP
counting on the 12 operational submarines. The Navy
may delay that date. NNSA had planned to retire the B83,
operates two bases for these submarines—one in Bangor,
the largest bomb remaining in the U.S. arsenal, around
WA, and one in Kings Bay, GA.
2025, after the completion of the B61 LEP, but, according
to the 2018 NPR, may retain it for a longer time.
The Navy plans to begin retiring Ohio-class submarines in
2027; it is producing the new Columbia-class submarine,
The Air Force is planning to replace the aging air-launched
which is expected to begin entering the fleet in 2031. The
cruise missiles carried by B-52 bombers with a new
Navy initially estimated that each new submarine could cost
advanced Long Range Standoff (LRSO) cruise missile.
$6 billion to $7 billion in FY2010 dollars, but it has worked
According to the Air Force, the existing ALCM has been
to redesign the submarine and reduce the costs. The Navy
through several life extension programs and is beginning to
has recently indicated that the fleet of 12 new submarines
show reliability problems. Reports indicate that the Air
will cost $139 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 also
State Department. New START Treaty. April 2010
initiated 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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