link to page 1 link to page 1 
Updated March 29, 2022
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 gravity bombs. The
The United States has reaffirmed the value of the nuclear
number of nuclear warheads carried on these delivery
triad. The Obama Administration noted, in the 2010
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 September 2021 indicated that the United
Triad leg to another if necessary due to unexpected
States had 1,389 warheads deployed on 665 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
According to press
bombers
reports, the Biden Administration NPR is likely to
B-2 bombers
20
12
12a
recommend continuing investments in the modernization
Total
800
675
1,467
programs for all three legs of the triad.
Source: U.S. Department of State. New START Treaty Aggregate
Current Forces and Modernization Plans
Numbers of Strategic Offensive Arms, December 1, 2020.
The United States is recapitalizing each leg of its nuclear
a. The treaty attributes one warhead to each deployed bomber,
triad and refurbishing many of the warheads carried by
although they could carry up to 20 bombs or cruise missiles.
those systems.
Rationale for the Triad
ICBMs
Early in the Cold War, the United States developed these
Before implementing the New START Treaty, the United
three types of nuclear delivery vehicles, in large part
States deployed 450 Minuteman III ICBMs at three Air
because each of the military services wanted to play a role
Force bases: F.E. Warren AFB in Wyoming, Malmstrom
in the U.S. nuclear arsenal. However, during the 1960s and
AFB in Montana, and Minot AFB in North Dakota. Under
1970s, analysts developed a more reasoned rationale for the
New START, the number has declined to 400 deployed
nuclear “triad.” They argued that these different basing
missiles, although the Air Force has retained all 450 silo
modes had complementary strengths and weaknesses that
launchers. While each Minuteman III missile originally
would enhance deterrence and discourage a Soviet first
carried three warheads, each now carries a single warhead,
strike. For example, ICBMs were believed to have the
both to reduce U.S. forces to New START levels and to
accuracy and prompt responsiveness needed to attack
adopt what is considered a more stabilizing posture.
hardened targets such as Soviet command posts and ICBM
silos, SLBMs had the survivability needed to complicate
https://crsreports.congress.gov
Defense Primer: Strategic Nuclear Forces
The Air Force has completed several programs designed to
missiles. It can also carry conventional weapons and has
improve the accuracy and reliability of the Minuteman fleet
participated in U.S. military campaigns from Bosnia to Iraq.
and to “support the operational capability ... through 2030.”
The Air Force maintains 76 B-52H bombers at two bases,
The Air Force is also developing a new ICBM, the Ground-
Barksdale, LA, and Minot, ND. The B-52 bomber, which
based Strategic Deterrent (GBSD), which will replace all
first entered service in 1961, is equipped to carry nuclear or
missiles and ground launch control facilities. It plans to
conventional air-launched cruise missiles (ALCMs). The B-
acquire 642 missiles to support testing and the deployment
52 bombers can also deliver a wide range of conventional
of a force of 400 missiles. The Air Force expects the
arms, and are currently receiving numerous upgrades to
program to reach its initial operational capacity, with 9
their communications and electronics systems. The Air
missiles on alert, by 2029; it expects to complete the
Force is also acquiring a new B-21 bomber, for both
deployment, with 400 missiles on alert, in 2036. The Biden
conventional and nuclear missions. It hopes to field
Administration has included $3.6 billion for the program in
between 80 and 100 of the new bombers, with the first to
its FY2023 budget request.
enter service around 2025. The Air Force has included $5
billion for this bomber in its FY2023 budget request.
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 is expected to begin to enter the
of 240 deployed launchers and around 1,000 warheads
force in 2022. NNSA had planned to retire the B83, the
counting on the 12 operational submarines. The Navy
largest bomb remaining in the U.S. arsenal, around 2025,
operates two bases for these submarines—one in Bangor,
after the completion of the B61 LEP. The 2018 NPR
WA, and one in Kings Bay, GA.
supported plans to retain this bomb, but press reports
indicate that the Biden Administration NPR may, again,
The Navy plans to begin retiring Ohio-class submarines in
support its retirement.
2027, with the new Columbia-class submarine expected to
begin entering the fleet in 2031. The Navy has recently
The Air Force is planning to replace the aging air-launched
indicated that the fleet of 12 new submarines will cost $139
cruise missiles carried by B-52 bombers with a new
billion. It has requested $6.3 billion for the Columbia-class
advanced Long Range Standoff (LRSO) cruise missile.
submarine in its FY2023 budget.
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 has pursued a life-extension program for the
Force plans to buy a total of 1,000-1,100 LRSO missiles, at
missiles, funded at about $1 billion per year, so that they
a cost of around $10.8 billion. It has included $1 billion for
will remain capable and reliable into the deployment of a
the missile in its FY2023 budget request. NNSA is also
new, Columbia-class submarine. It has also initiated a
conducting a life-extension program on the W80 warhead to
second life extension program for these missiles, known as
provide a warhead for the new LRSO.
the D5LE II, and has included $173 million in its budget
request for FY2023.
CRS Products
NNSA is also working to extend the life of the warheads
CRS Report R41219, The New START Treaty: Central Limits and
carried by U.S. SLBMs. It has conducted a life extension
Key Provisions, by Amy F. Woolf
program (LEP) for the W76 warhead, which is carried by
CRS Report RL33640, U.S. Strategic Nuclear Forces: Background,
most Trident missiles, and provided a small number of low-
Developments, and Issues, by Amy F. Woolf
yield warheads, known as the W76-2, to the Navy in late
2019. NNSA is also altering the W88 warhead, which is
carried by a portion of the fleet, to address concerns with its
safety and reliability. In particular, the program will replace
Other Resources
the aging arming, fuzing, and firing components. It has also
initiated work on the W93 warhead, which will eventually
DOD. Nuclear Posture Review Report. April 2010.
deploy on D-5 missiles.
State Department. New START Treaty. April 2010
Heavy Bombers
The Air Force has 20 B-2 bombers, based at Whiteman
AFB in Missouri. The B-2 bomber can carry both B61 and
Amy F. Woolf, Specialist in Nuclear Weapons Policy
B83 nuclear bombs, but it is not equipped to carry cruise
https://crsreports.congress.gov
Defense Primer: Strategic Nuclear Forces
IF10519
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 11 · UPDATED