From the Introduction: "The U.S. military has a long history of providing support to civil authorities, particularly in response to disasters, but for other purposes as well. The Defense Department currently defines 'defense support of civil authorities' as 'Support provided by U.S. Federal military forces, DOD civilians, DOD contract personnel, DOD Component assets, and National Guard forces (when the Secretary of Defense, in coordination with the Governors of the affected States, elects and requests to use those forces in title 32, U.S.C., status) in response to requests for assistance from civil authorities for domestic emergencies, law enforcement support, and other domestic activities, or from qualifying entities for special event.' (DOD Directive 3025.18)."
CRS In Focus, IF11324
Congressional Research Service: https://crsreports.congress.gov/