On the southwest fringes of Chicago, Texas National Guard soldiers toting M4 rifles and dressed in fatigues set up camp for an indefinite period on the ground. President Donald Trump authorized the mobilization, despite resistance from the city's mayor and the governor of Illinois to this move by the president and Homeland Security Secretary Kristi Noem.

"Probably worse than almost any city in the world," Trump said of Chicago.

The deployment is another step in "this march toward autocracy," Illinois Gov. JB Pritzker said. "The state of Illinois is going to use every lever at our disposal to resist this power grab and get Noem's thugs the hell out of Chicago."

The commander in chief responded that the governor and the mayor should be jailed.

From Chicago to California, from the District of Columbia to Oregon, Trump has ordered troops into cities led by Democrats, places he has labeled crime dens. These deployments, according to Trump, are intended to tamp down public demonstrations against arrests and raids that stem from his immigration policies, as well as to quell crime and violence in those cities.

Now, with courts blocking some of the deployments, Trump is publicly weighing another option: He said he may invoke the Insurrection Act to get around judicial orders.

Using the two century-old law to justify putting U.S. troops on American streets would be a provocative act, one that people across the political spectrum have equated to declaring war on U.S. citizens. The last time a president invoked the law was 1992, when federal troops deployed to Los Angeles to help control widespread riots following the acquittal of police officers accused of beating Black motorist Rodney King.

"The Trump administration has, for the past eight months, been inserting the military into routine law enforcement in a number of ways that have no real precedent in American history," Joseph Nunn, a counsel in the Brennan Center's Liberty and National Security Program, told Straight Arrow News. "We have never had a president seek to use the military this frequently and for this many purposes at home when there is no crisis to justify any of it."

The White House, however, insists people protesting the administration's immigration policies are staging an "insurrection."

"This is an all-out campaign of insurrection against the sovereignty of the United States because the Democrat Party and those who are committing violence in this country do not believe in the legitimacy of the sovereign territory of the United States, and they don't want any of these illegal aliens to go home," Stephen Miller, the deputy White House chief of staff, told reporters earlier this week.

President Thomas Jefferson signed the Insurrection Act into law in 1807. It allows Congress to "provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions."

The law gives the president, as commander in chief, some discretion to decide whether people deemed insurrectionists are causing mayhem and whether military force is needed to protect lives. States, too, can request the president to invoke the Insurrection Act, as California Gov. Pete Wilson did in 1992.

"We have an Insurrection Act for a reason," Trump said in the Oval Office filled with reporters and cameras on Monday. "If I had to enact it, I'd do that. If people were being killed, and courts were holding us up, or governors or mayors were holding us up, sure, I'd do that."

Some legal scholars say the Insurrection Act is vague and needs revision.

"The idea behind the Insurrection Act is in the event of an emergency -- an insurrection, a rebellion, massive violence, civil unrest -- we want the president to be able to call on the military to assist civilian authorities in restoring order," Nunn told SAN.

But, Nunn added, the law is too broad. "It grants the president too much discretion as to when it's okay to invoke it, and there aren't sufficient procedural guardrails against abuse. It's not a blank check, but it is dangerously broad, and it's something that Congress urgently needs to reform.

This act is rarely used -- just 30 times in U.S. history, according to the Brennan Center. In the 1950s and 1960s, Presidents Dwight Eisenhower, John Kennedy and Lyndon Johnson invoked the act to deal with violent opposition to the civil rights movement. All three acted against the wishes of the governors of Alabama, Arkansas and Mississippi. In 1992, President George H.W. Bush waited for California's governor and the mayor of Los Angeles to request federal help before sending troops during the Rodney King riots.

Trump, too, sent troops to Los Angeles earlier this year after demonstrators challenged aggressive immigration raids that supported the president's pledge for mass deportations of immigrants without legal status. Citing Section 12406 of Title X of the U.S. Code, Trump nationalized the California National Guard, over the objections of California Gov. Gavin Newsom. He dispatched 4,200 National Guard soldiers and 700 Marines into Los Angeles.

"Title X is the body of statutes that govern the military," Nunn said. "The Insurrection Act is also part of Title X. He didn't use the Insurrection Act. Instead, he has used the statute, 12406, that is not historically understood as an authority that allows the president to deploy troops. It's instead always been seen as a federalization authority."

The Trump administration, however, has argued in court that the statute both approves deployment and acts as an exception to the Posse Comitatus Act, the law that bars federal troops from participating in law enforcement unless authorized by Congress.

"So the Trump administration's interpretation of this law is unprecedented," Nunn said. "It's novel. It's not consistent with how the statute has always been understood."

Trump has faced strong opposition from mayors and governors -- as well as judges -- as he has issued a series of orders that assigned the military to law enforcement duties.

In August, he called up 800 troops from the District of Columbia National Guard after declaring a crime emergency in Washington. Local officials pointed out that the crime rate had declined significantly, but they had little recourse in stopping the deployment.

"The president has a freer hand in D.C. because it's not a state, because it's the seat of the federal government, than he does anywhere else," Nunn said.

But last weekend, U.S. District Judge Karin Immergut, a Trump appointee, twice blocked the administration from sending National Guard troops into Portland, Oregon. Trump said the city has been "on fire for years," adding "I think that's all insurrection, really criminal insurrection."

Immergut's first order blocked the deployment of Oregon National Guard troops. Then Trump attempted to send troops from neighboring California. Immergut told the Department of Justice the second deployment was "in direct contravention" of her restraining order.

Some advisers reportedly urged Trump to declare martial law after he refused to accept the results of the 2020 presidential election. And some current aides have suggested Trump take strong action to put down what they claim is an insurrection by people protesting immigration policies.

While martial law and the Insurrection Act can be conflated, these are two separate concepts.

"The two terms are dramatically different, and they are frequently confused," Nunn said. "All that the Insurrection Act allows the president to do is use the military to suppress insurrections and to execute federal law... Martial law is a very different thing."

U.S. law does not expressly define martial law.

"It's been the subject of confusion and debate for basically the entirety of American history," Nunn said. "What that term usually refers to though is an emergency power that allows the military to push aside civilian government and exercise jurisdiction directly over civilians. ... It puts the military in charge of everything."

Supplanting civilian government with military rule is impossible to reconcile with the Constitution, Nunn said. "It goes against, you know, every basic principle of our constitutional system."