Providing Continued Federal Support for Governors' Use of the National Guard to Respond to COVID-19 and to Facilitate Economic Recovery: Memorandum for the Secretary of Defense [And] the Secretary of Homeland Security, April 20, 2020 [open pdf - 240KB]
From the document: "By the authority vested in me as President by the Constitution and the laws of the United States of America, including the Robert T. Stafford Disaster Relief and Emergency Assistance Act, 42 U.S.C. 5121-5207 (the 'Stafford Act'), and section 502 of title 32, United States Code, it is hereby ordered as follows: [...] It is the policy of the United States to take measures to assist State and territorial Governors under the Stafford Act in their responses to all threats and hazards to the American people in their respective States and territories. On March 13, 2020, I declared a national emergency recognizing the threat that COVID-19 [coronavirus disease 2019], the disease caused by the novel (new) coronavirus known as SARS-CoV-2 [severe acute respiratory syndrome coronavirus 2] ('the virus'), and the virus poses to the Nation's healthcare systems. I also determined that same day that the COVID-19 outbreak constituted an emergency, of nationwide scope, pursuant to section 501(b) of the Stafford Act (42 U.S.C. 5191(b)). Considering the profound and unique public health risks posed by the ongoing outbreak of COVID-19, the need for close cooperation and mutual assistance between the Federal Government and the States and territories is greater than at any time in recent history. This need remains as the United States continues to battle the public health threat posed by the virus, while transitioning to a period of increased economic activity and recovery in those areas of the Nation where the threat posed by the virus has been sufficiently mitigated."
Federal Register: http://www.gpo.gov/fdsys/browse/collection.action?collectionCode=FR
Federal Register (April 24, 2020), v.85 no.80, p.23203