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SCongressional Research Service
Inforrning the legislative debate since 1914
S
Updated April 9, 2025
Defense Primer: Defense Support of Civil Authorities
Introduction
Throughout its history, the U.S. military has assisted
federal, state, and local authorities in responding to
homeland security incidents when the response
requirements exceeded those authorities' resources. Such
assistance is called Defense Support of Civil Authorities
(DSCA). Situations where the Department of Defense
(DOD) has provided DSCA include
* Border security,
* Civil disturbances,
* Natural disasters (e.g., fires, hurricanes),
* Public health emergencies (e.g., COVID-19 response),
* Oil spills, and
* Special events, (e.g., presidential inaugurations).
Common tasks for military personnel performing DSCA
include transporting supplies, clearing or constructing
roads, controlling traffic, and conducting surveillance.
DOD's Role in Homeland Security Activities
DOD is the primary agency for homeland defense (HD)
activities, defined in law as activity undertaken for the
military protection of the territory or domestic population of
the United States, or of infrastructure or other assets of the
United States determined by the Secretary of Defense as
being critical to national security, from a threat or
aggression against the United States. DOD is typically a
supporting agency for homeland security (HS) activities, of
which DSCA is a subset (see Figure 1). DOD's role
depends on the magnitude of the threat. According to DOD,
The characterization of a particular threat [as HD or HS],
and the designated response agencies and modes, ultimately
rests with the President. (Joint Publication 3-27, 2018.)
DOD policy stipulates that HS incidents be managed at the
lowest level possible, be that at the state, local, territorial, or
tribal level. When an incident exceeds, or is anticipated to
exceed, the resources of these entities, the U.S. government
may provide resources and capabilities to support the
response, including predicting, assessing, preparing for, and
recovering from domestic incidents.
The U.S. military does not have units specifically dedicated
to DSCA. Rather, DOD derives DSCA capabilities from its
warfighting capabilities and provides them to civil
authorities on a reimbursable basis. Further, DOD
components are not specifically funded to train, equip, and
exercise for DSCA operations. Still, DOD policy and
actions indicate that the department proactively participates
in planning for homeland security incidents and
incorporates DSCA in exercises and training. (DOD
Directive (DODD) 3025.18, 2018.)
Figure I. Relationship between Homeland Defense,
Homeland Security, and DSCA
________DOD Lead Role
DeZfense S 1)r - DODSupport
iCivil Autioritte Role
- Executedatall
S;nneurty' tinmesunder
supportedagency
authorities
Source: Derived from DOD, JP 3-27, 2018, Fig. A-1, p. A-2.
Relevant Statutory Authorities
The coordination of national preparedness activities,
including DOD's role in these efforts, is set forth in
national planning frameworks and implemented by relevant
agencies. Laws guiding DOD's activities include
The Robert T. Stafford Disaster Relief and Emergency
Assistance Act (42 U.S.C. §§5121-5207, Stafford Act) is
the statutory authority for relief from domestic natural
disasters and some human-caused incidents. It authorizes
the President to declare an incident a major disaster or an
emergency and authorizes certain types of federal
assistance to state, local, tribal, and territorial governments.
The Economy in Government Act (31 U.S.C. §1535,
Economy Act) provides authority for federal agencies to
order goods or services from other federal agencies
provided the ordered goods or services cannot be provided
by contract as conveniently or cheaply by a commercial
enterprise. This act permits an agency to request DOD
support in situations other than those outlined in the
Stafford Act.
Military Support for Civilian Law Enforcement
Agencies provisions (10 U.S.C. §§271-284) authorize
certain types of military support to civilian law enforcement
officials and agencies. In specified circumstances, support
may include use of military equipment and facilities,
training and advising, maintenance and operation of
equipment, support for counterdrug activities, and support
for activities to counter transnational organized crime.
The Posse Comitatus Act (18 U.S.C. §1385) prohibits
any part of the Army, the Navy, the Marine Corps, the Air
Force, or the Space Force from being involved in civilian
law enforcement except when expressly authorized by the
Constitution or Congress. Courts have generally construed
this act to prohibit activities such as search, seizure, and
arrest.
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