Per the Wikipedia:
The Act empowers the U.S. president to call into service the U.S. Armed Forces and the National Guard:
- when requested by a state's legislature, or governor if the legislature cannot be convened, to address an insurrection against that state (§ 251),
- to address an insurrection, in any state, which makes it impracticable to enforce the law (§ 252), or
- to address an insurrection, domestic violence, unlawful combination or conspiracy, in any state, which results in the deprivation of Constitutionally-secured rights, and where the state is unable, fails, or refuses to protect said rights (§ 253).
[...]
In 2016, Public Law 114-328 was amended [...]. §252: "Use of militia and armed forces to enforce Federal authority" currently reads:
Whenever the President considers that unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States, make it impracticable to enforce the laws of the United States in any State by the ordinary course of judicial proceedings, he may call into Federal service such of the militia of any State, and use such of the armed forces, as he considers necessary to enforce those laws or to suppress the rebellion.[9][6]
[...]
On June 1, 2020, President Donald Trump warned that he would invoke the Act in response to the violent demonstrations following the death of George Floyd.[13][14][15] In his official June 1 statement, President Trump urged "every governor to deploy the National Guard in sufficient numbers" to re-establish civil law and order "until the violence has been quelled"; however, military forces were never deployed.[16]