Providing Federal Support for Governors' Use of the National Guard to Respond to COVID-19: Memorandum for the Secretary of Defense [And] the Secretary of Homeland Security, April 7, 2020 [open pdf - 246KB]
From the Document: "It is the policy of the United States to take measures to assist State Governors under the Stafford Act in their responses to all threats and hazards to the American people in their respective States. Considering the profound and unique public health risks posed by the ongoing outbreak of COVID-19 [coronavirus disease 2019], the disease caused by the novel (new) coronavirus known as SARS-CoV-2 (''the virus''), the need for close cooperation and mutual assistance between the Federal Government and the States is greater than at any time in recent history. [...] All States have activated their Emergency Operations Centers and are working to fight the spread of the virus and attend to those who have symptoms or who are already infected with COVID-19."
Federal Register: http://www.gpo.gov/fdsys/browse/collection.action?collectionCode=FR
Federal Register (April 10, 2020), v.85 no.70, p.20383