Insurrection Act explained: How 1807 law could be used as Trump deploys National Guard in Chicago

CHICAGO (WLS) -- For weeks, President Donald Trump has threatened to send the National Guard to Chicago. He is now doing it, and is considering using a law enacted more than two centuries ago to legally justify the deployment.

Troops from Texas have arrived. For now, they are gearing up at a U.S. Army Reserve facility in Elwood. While their exact mission is unclear, what is clear is that Gov. JB Pritzker does not want the guard in his state. He is taking legal action to prevent it. So, President Trump may invoke the Insurrection Act if a federal court prevents him from deploying the Guard.

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"That is an act that was passed by Congress in 1807, back in colonial days, when there was a concern about possible insurrections among, within the states," retired Illinois National Guard Maj. Gen. David Harris said.

Harris says the Insurrection Act allows troops to make arrests. Rarely enacted, it has been used to quell major disturbances or uprisings.

"There are two scenarios in which the Insurrection Act can be invoked and federal and National Guard troops can be sent. One is when the governor requests them," said Professor Daniel Rodriguez, with Northwestern Pritzker School of Law.

In the early 90s, the California governor requested the National Guard under the Insurrection Act to quell the Rodney King Riots.

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The other scenario is the president initiates the statute. President Lyndon Johnson did it during the 1960s to enforce the Civil Rights Act.

"If I had to enact it, I do. I do that if people were being killed, and courts holding this up, or governors and mayors holding it up, sure. I do that," Trump said.

But, legal experts say mayors and governors taking legal action, isolated crimes or Mayor Brandon Johnson's executive order establishing United States Immigration and Customs Enforcement-free zones are likely not reasons to invoke the Insurrection Act. The statute has only been used for major civil unrest.

"The Insurrection Act just is not contemplated to be a way, it could basically say, 'Oh, wow, there could possibly be a crime that would happen, or some threat to ICE agents, and we need to send it in, federal troops,'" Rodriguez said.

If a federal judge rules in favor of Pritzker, and the president decides to use the Insurrection act, the statute says he must issue a proclamation before deploying troops to the streets of Chicago.