I asked Google what the Insurrection Act is and this is what I got:

The Insurrection Act is a U.S. federal law from 1807 that authorizes the President to deploy U.S. military forces domestically to suppress civil disorder, insurrection, or rebellion. This law is an exception to the Posse Comitatus Act of 1878, which generally restricts the use of federal military personnel for civilian law enforcement.

The President can invoke the Act under specific circumstances outlined in Title 10 of the U.S. Code: When a state requests federal help to suppress an insurrection or domestic violence.

When "unlawful obstructions, combinations, or assemblages, or rebellion" make it impossible to enforce federal laws through normal judicial means. When domestic violence or conspiracy in a state deprives people of their constitutional rights, and state authorities cannot or will not protect those rights.

Historically, the Act has been used during significant national crises, including the Civil War, Reconstruction, the Civil Rights Movement, and the 1992 Los Angeles riots. The last formal invocation was in 1992 by President George H.W. Bush at the request of the California governor.

Legal experts point out that the Act's broad language gives the President considerable discretion in deciding when to invoke it, leading to discussions about potentially adding clearer limits and congressional oversight. Invoking the Insurrection Act permits the military to perform law enforcement duties such as making arrests, but it does not suspend the Constitution or impose martial law.