https://crsreports.congress.gov
Updated March 29, 2022
Defense Primer: Strategic Nuclear Forces
The Nuclear Triad
Since the early 1960s, the United States has maintained a
“triad” of strategic nuclear delivery vehicles. These include
long-range land-based intercontinental ballistic missiles
(ICBMs), long-range submarine-launched ballistic missiles
(SLBMs), and long-range heavy bombers equipped to carry
nuclear-armed cruise missiles and gravity bombs. The
number of nuclear warheads carried on these delivery
vehicles peaked in the late 1980s, at around 14,000
warheads. It has been declining ever since, both as the
United States complies with limits in U.S.-Russian arms
control agreements and as it has changed requirements after
the Cold War. As of February 2018, the United States had
reduced its forces to comply with the New START Treaty,
which entered into force in early 2011. Table 1 displays the
U.S. forces that count under the treaty limits, as of
September 1, 2020. These forces fall below the treaty limits
of 1,550 deployed warheads on 700 deployed missiles due
to maintenance schedules and operational requirements. An
update released in September 2021 indicated that the United
States had 1,389 warheads deployed on 665 missiles and
bombers.
Table 1. U.S. Strategic Nuclear Forces Under New
START
Source: U.S. Department of State. New START Treaty Aggregate
Numbers of Strategic Offensive Arms, December 1, 2020.
a. The treaty attributes one warhead to each deployed bomber,
although they could carry up to 20 bombs or cruise missiles.
Rationale for the Triad
Early in the Cold War, the United States developed these
three types of nuclear delivery vehicles, in large part
because each of the military services wanted to play a role
in the U.S. nuclear arsenal. However, during the 1960s and
1970s, analysts developed a more reasoned rationale for the
nuclear “triad.” They argued that these different basing
modes had complementary strengths and weaknesses that
would enhance deterrence and discourage a Soviet first
strike. For example, ICBMs were believed to have the
accuracy and prompt responsiveness needed to attack
hardened targets such as Soviet command posts and ICBM
silos, SLBMs had the survivability needed to complicate
Soviet efforts to launch a disarming first strike and to
retaliate if such an attack were attempted, and heavy
bombers could be dispersed quickly and launched to
enhance their survivability, and they could be recalled to
their bases if a crisis did not escalate into conflict.
The United States has reaffirmed the value of the nuclear
triad. The Obama Administration noted, in the 2010
Nuclear Posture Review (NPR), that the unique
characteristics of each leg of the triad were important to the
goal of maintaining strategic stability at reduced numbers of
warheads. It pointed out that strategic nuclear submarines
(SSBNs) are the most survivable leg of the triad, that
single-warhead ICBMs contribute to stability and are not
vulnerable to air defenses, and that bombers can be
deployed as a signal in crisis, to strengthen deterrence and
provide assurances to allies and partners. It also noted that
“retaining sufficient force structure in each leg to allow the
ability to hedge effectively by shifting weight from one
Triad leg to another if necessary due to unexpected
technological problems or operational vulnerabilities.”
The Trump Administration also reaffirmed the U.S.
commitment to the nuclear triad and to the modernization
programs for each of the components of that force structure.
It noted in the 2018 NPR that “the triad’s synergy and
overlapping attributes help ensure the enduring
survivability of our deterrence capabilities against attack
and our capacity to hold a range of adversary targets at risk
throughout a crisis or conflict. Eliminating any leg of the
triad would greatly ease adversary attack planning and
allow an adversary to concentrate resources and attention
on defeating the remaining two legs.” According to press
reports, the Biden Administration NPR is likely to
recommend continuing investments in the modernization
programs for all three legs of the triad.
Current Forces and Modernization Plans
The United States is recapitalizing each leg of its nuclear
triad and refurbishing many of the warheads carried by
those systems.
ICBMs
Before implementing the New START Treaty, the United
States deployed 450 Minuteman III ICBMs at three Air
Force bases: F.E. Warren AFB in Wyoming, Malmstrom
AFB in Montana, and Minot AFB in North Dakota. Under
New START, the number has declined to 400 deployed
missiles, although the Air Force has retained all 450 silo
launchers. While each Minuteman III missile originally
carried three warheads, each now carries a single warhead,
both to reduce U.S. forces to New START levels and to
adopt what is considered a more stabilizing posture.
The Air Force has completed several programs designed to
improve the accuracy and reliability of the Minuteman fleet
and to “support the operational capability ... through 2030.”
The Air Force is also developing a new ICBM, the Ground-
