But what sort of authority does it give Trump? Here is the rundown:

Historical context: The Insurrection Act of 1807 is one of the country's earliest laws, according to the Congressional Research Service. Congress "authorized the president to call out the militia, initially to protect the frontier against 'hostile incursions of the Indians,'" the report says. "Insurrections against state governments could be putdown under the act only if the state legislature applied for such assistance."
    The nation's third president, Thomas Jefferson, signed the act to foil a plot by Aaron Burr, recounts The History Channel website. After killing Alexander Hamilton in a dual in 1804, Burr had turned his energies westward and planned to annex Spanish territory in Louisiana and Mexico to establish an independent republic. 
     Jefferson, alerted to the plot, asked Secretary of State James Madison if the Constitution granted authority to the president to deploy the army to stop the insurrection. Madison said no.
     Jefferson then took another tack, releasing a proclamation that warned, "sundry persons, citizens of the United States or residents within the same, are conspiring and confederating together to begin and set on foot, provide, and prepare the means for a military expedition or enterprise against the dominions of Spain; that for this purpose they are fitting out and arming vessels in the western waters of the United States."  He asked officers to bring to "punishment all persons engaged or concerned in such enterprise."
     During this episode, Jefferson did only what the Constitution said he could do, but he also asked Congress to pass a bill to authorize use of the military in case of an insurrection. By the time the bill was signed, Burr was in custody, the History Channel website reports.

What the Insurrection Act does: The act "is the principal authority relied upon by the president when deploying troops within the United States in response to a domestic emergency," writes Thaddeus Hoffmeister in the Stetson Law Review. "The Act serves as the major exception to the Posse Comitatus Act, which prohibits federal military forces from actively participating in civilian law enforcement absent congressional or constitutional authority. When invoked by the president, the Insurrection Act allows federal military forces to participate in domestic law-enforcement activities."

Changes: After the Civil War, Congress added a provision for the use of federal military forces to protect civil rights.

How does it work? The president can deploy the military under certain circumstances, according to the blog Lawfare:

• If the president receives a request to do so by the legislature of a state undergoing an insurrection;
• If it is impractical to go through the judicial system;
• If any group of people is "deprived of a right, privilege, immunity, or of any protection named in the Constitution and secured by law," and the state fails to protect that right. 
• If any "insurrection, domestic violence, unlawful combination, or conspiracy ... opposes or obstructs the execution of the laws of the United States or impedes the course of justice under those laws.”

The present context: Trump called for “the deployment of thousands and thousands of heavily armed soldiers, military personnel, and law enforcement officers to stop the rioting, looting, vandalism, assaults, and the wanton destruction of property.”
     The unrest developed in response to the murder of George Floyd, a 46-year-old unarmed African American man taken into custody by Minneapolis police. The officer who pressed his knee against Floyd's neck has been charged with second-degree murder, and three other officers were charged with aiding and abetting second-degree murder and aiding and abetting second-degree manslaughter. But the incident brought to a head anger about the use of force by police in a series of incidents nationwide.
    Trump's advisers disagreed about whether to use the military against the American people. By Thursday, news outlets reported that Washington, D.C., Mayor Muriel Bowser wanted out-of-state troops out of the city. And as the days passed, demonstrations were largely peaceful. 
    But news outlets reported that some officers in unmarked uniforms wouldn't respond to questions about where they were from.

     Resources:

• CNN: DC mayor says she wants out-of-state troops out of Washington, By Veronica Stracqualursi and Nicky Robertson, June 4, 2020.
  
• Congressional Research Service: Defense Primer-- Legal Authorities for the Use of Military Forces, Jan. 3, 2020.
• Foreign Policy: The Pentagon Distances Itself from Trump, by Jack Detsch, June 3, 2020.
• The History Channel: Thomas Jefferson Signed the Insurrection Act in 1807 to Foil a Plot by Aaron Burr, By David Roos, June 3, 2020.
  
• The History Channel: Aaron Burr arrested for alleged treason, This Day in History, Feb. 19, 1807.
  
• Lawfare blog: Can Trump Use the Insurrection Act to Deploy Troops to American Streets? By Scott R. Anderson and Michel Paradis, June 3, 2020.
  
• Stetson Law Review: An Insurrection Act for the 21st Century, by  Thaddeus Hoffmeister.
• University of Virginia, Miller Center: Nov. 27, 1806, Proclamation on Spanish Territory.
• The White House: Statement by the president, June 1, 2020.

     Related:

     Hamilton essay: What it takes to be president.
    

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