link to page 1 link to page 1 
Updated December 11, 2018
Defense Primer: Strategic Nuclear Forces
The Nuclear Triad
enhance their survivability, and they could be recalled to
Since the early 1960s the United States has maintained a
their bases if a crisis did not escalate into conflict.
“triad” of strategic nuclear delivery vehicles. These include
long-range land-based intercontinental ballistic missiles
The United States has reaffirmed the value of the nuclear
(ICBMs), long-range submarine-launched ballistic missiles
triad. The Obama Administration noted, in the 2010
(SLBMs), and long-range heavy bombers equipped to carry
Nuclear Posture Review (NPR), that the unique
nuclear-armed cruise missiles and nuclear-armed gravity
characteristics of each leg of the triad were important to the
bombs. The number of nuclear warheads carried on these
goal of maintaining strategic stability at reduced numbers of
delivery vehicles peaked in the late 1980s, at around 14,000
warheads. It pointed out that strategic nuclear submarines
warheads. It has been declining ever since, both as the
(SSBNs) are the most survivable leg of the triad, that
United States complies with limits in U.S.-Russian arms
single-warhead ICBMs contribute to stability and are not
control agreements and as it has changed requirements after
vulnerable to air defenses, and that bombers can be
the Cold War. At the present time, the United States is
deployed as a signal in crisis, to strengthen deterrence and
reducing its forces to comply with the New START Treaty,
provide assurances to allies and partners. It also noted that
which entered into force in early 2011. Table 1 displays the
“retaining sufficient force structure in each leg to allow the
force structure that the United States plans to retain when
ability to hedge effectively by shifting weight from one
that treaty’s limits become binding in February 2018.
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 will retain 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
silo-based ICBMs but replace the entire flight system and
bombers could be dispersed quickly and launched to
https://crsreports.congress.gov
Defense Primer: Strategic Nuclear Forces
all the ground launch control facilities. The Air Force plans
B83 nuclear bombs, but it is not equipped to carry cruise
to acquire 642 missiles to support testing and the
missiles. It can also carry conventional weapons and has
deployment of a force of 400 missiles. The Air Force has
participated in U.S. military campaigns from Bosnia to Iraq.
estimated that this program will cost $62.3 billion over 30
The Air Force maintains 76 B-52H bombers at two bases,
years, although the Pentagon’s Cost Assessment and
Barksdale, LA, and Minot, ND. The B-52 bomber, which
Program Evaluation Office has estimated that the cost could
first entered service in 1961, is equipped to carry nuclear or
reach $85 billion over 30 years. The Air Force expects the
conventional air-launched cruise missiles (ALCMs). The B-
program to reach its initial operational capacity, with 9
52 bombers can also deliver a wide range of conventional
missiles on alert, by 2029; it expects to complete the
arms, and are currently receiving numerous upgrades to
deployment, with 400 missiles on alert, in 2036.
their communications and electronics systems. The Air
Force is also acquiring a new B-21 bomber, for both
SLBMs
conventional and nuclear missions. It hopes to field
The United States currently has 14 Trident (Ohio-class)
between 80 and 100 of the new bombers, with the first to
ballistic missile submarines, with two in overhaul and 12 in
enter service around 2025. Costs remain classified.
the operational fleet. Under the New START Treaty, each
of the submarines carries only 20, rather than the original
According to unclassified estimates, the United States has
24, missiles. Using treaty counting rules, the 14 submarines
around 475 B61 and B83 bombs. Several variants of the
count as a total of 280 deployed and nondeployed
B61 bomb are undergoing a life extension program (LEP),
launchers, with 240 deployed launchers and around 1,000
to enhance the bomb’s safety, security, and use control
warheads counting on the 12 operational submarines. The
features. The Air Force is also designing a new tail kit for
Navy operates two bases for these submarines—one in
the B61 bomb, replacing the parachute currently used to
Bangor, WA, and one in Kings Bay, GA.
slow the bomb’s descent, to improve its accuracy. NNSA
estimates that the B61 LEP will cost nearly $9 billion, with
The Ohio-class submarines are scheduled to begin retiring
an additional $1-$2 billion in Air Force funding for the new
from the fleet in 2027; the Navy is designing a new
tailkit. The new model is expected to begin to enter the
Columbia-class submarine that it expects to begin entering
force in 2020. NNSA had planned to retire the B83, the
the fleet in 2031. The Navy initially estimated that each
largest bomb remaining in the U.S. arsenal, around 2025,
new submarine could cost $6 billion to $7 billion in
after the completion of the B61 LEP, but, according to the
FY2010 dollars, but it has worked to redesign the
2018 NPR, may retain it for a longer time.
submarine and reduce the costs, with the plan to hold each
submarine to around $4.9 billion, in FY2010 dollars. The
The Air Force is planning to replace the aging air-launched
Navy has recently indicated that, using then-year dollars
cruise missiles carried by B-52 bombers with a new
rather than 2010 dollars, the fleet of 12 new submarines
advanced long range standoff (LRSO) cruise missile.
will cost $139 billion. Deployment will begin in 2031.
According to the Air Force, the existing ALCM has been
through several life extension programs and is beginning to
The Navy purchased over 530 D-5 missiles to support the
show reliability problems. Reports indicate that the Air
Trident fleet. It is now pursuing a life-extension program
Force plans to buy a total of 1,000-1,100 LRSO missiles, at
for the missiles, so that they will remain capable and
a cost of around $10.8 billion, to support the testing
reliable throughout the life of the Ohio-class submarines
program and deployment plans over the life of the missile.
and into the deployment of a new, Columbia-class
NNSA is also conducting a life-extension program on the
submarine. The Navy currently spends approximately $1
W80 warhead to provide a warhead for the new LRSO.
billion per year on this life extension program.
CRS Products
The National Nuclear Security Administration (NNSA), a
semi-autonomous agency in the Department of Energy, is
CRS Report R41219, The New START Treaty: Central Limits and
also working to extend the life of the warheads carried by
Key Provisions, by Amy F. Woolf
U.S. SLBMs. NNSA plans to complete a life extension
CRS Report RL33640, U.S. Strategic Nuclear Forces: Background,
program (LEP) for the W76 warhead, which is carried by
Developments, and Issues, by Amy F. Woolf
most Trident missiles, by 2019. This LEP is intended to add
30 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
Other Resources
components.” It will also include a small number of low-
yield warheads, known as the W76-2, as mandated by the
DOD. Nuclear Posture Review Report. April 2010.
2018 NPR. NNSA is also altering the W88 warhead, which
State Department. New START Treaty. April 2010
is carried by a portion of the fleet, to address concerns with
its safety and reliability. In particular, the program will
replace the aging arming, fuzing, and firing components.
Heavy Bombers
Amy F. Woolf, Specialist in Nuclear Weapons Policy
The Air Force has 20 B-2 bombers, based at Whiteman
IF10519
AFB in Missouri. The B-2 bomber can carry both B61 and
https://crsreports.congress.gov
Defense Primer: Strategic Nuclear Forces
Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff to
congressional committees and Members of Congress. It operates solely at the behest of and under the direction of Congress.
Information in a CRS Report should not be relied upon for purposes other than public understanding of information that has
been provided by CRS to Members of Congress in connection with CRS’s institutional role. CRS Reports, as a work of the
United States Government, are not subject to copyright protection in the United States. Any CRS Report may be
reproduced and distributed in its entirety without permission from CRS. However, as a CRS Report may include
copyrighted images or material from a third party, you may need to obtain the permission of the copyright holder if you
wish to copy or otherwise use copyrighted material.
https://crsreports.congress.gov | IF10519 · VERSION 5 · UPDATED