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). Defense support of civil authorities in response to disasters is typically carried out in accordance with the National Response Framework (NRF), which is a structure of preparedness that guides the nation in responding to domestic disasters and emergencies. The NRF is always in effect and its structures, roles, and responsibilities can be partially or fully implemented in response to a threat or hazard. It aims to produce a scaled response with appropriate coordination. Under the NRF framework, local and state governments are expected to put forth their best effort during incidents within their jurisdiction. They should only request federal assistance when their resources are overwhelmed. At that point, DOD may provide support in response to the Request for Assistance (RFA), typically as part of a broader federal response."
CRS In Focus, IF11324
Congressional Research Service: https://crsreports.congress.gov/