After President Donald Trump announced Monday that he is ready to send U.S. troops into states to quell riots, law professors at Catholic universities said acting against the wishes of state governors would be counterproductive, but likely would not violate the law.

Mass protests and some riots have occurred in major U.S. cities and suburbs since shortly after the May 25 death of George Floyd in Minneapolis. While 23 states have mobilized the National Guard to quell rioters, on June 1 Trump said he would deploy U.S. troops in states that have not done so.

Legal experts told CNA that the Insurrection Act, a law approved by Congress in 1807 and amended over the years, that allows the president to use the military on American soil in times of insurrection. But the statute is subject to a number of limitations, they said, and has historically been used in cooperation with state governors and not against their wishes.

Professor Mary Ellen O'Connell at the University of Notre Dame Law School said Trump "has a narrow statutory right" to send the U.S. military into states, but that in her judgment, "the right does not apply to the current civil unrest."

Trump cited the need to maintain law and order during a period of unrest, O'Connell said, but the current protests and riots have been created, in part, because of recklessness by law enforcement through "militarized policing."

"Our nation is witnessing the impact of excessive force, not the lack of it. Excessive force was used against George Floyd. It is being used by some seeking justice for him. It will not end with the deployment of military fire power," O'Connell said.

In the White House Rose Garden on Monday, President Trump said that he would deploy the U.S. military to states to quell riots, if state governors did not call up the National Guard.

"Today, I have strongly recommended to every governor to deploy the National Guard in sufficient numbers that we dominate the streets," Trump said, calling for "an overwhelming law enforcement presence until the violence has been quelled."

"If a city or a state refuses to take the actions that are necessary to defend the life and property of their residents, then I will deploy the United States military and quickly solve the problem for them," he said.

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In Washington, D.C., Trump said he would dispatch "thousands and thousands of heavily armed soldiers, military personnel, and law enforcement officers." Citing the vandalism of monuments and businesses and acts of violence against police, Trump called them "acts of domestic terror."

On Tuesday night the AP reported that " roughly 700 members of the Army's 82nd Airborne Division had arrived at two military bases near Washington. Another 1,400 soldiers are ready to be mobilized within an hour, the two Pentagon officials said. The soldiers are armed and have riot gear as well as bayonets."

Professor Antonio Fidel Perez of the Columbus School of Law at the Catholic University of America said that while Trump had threatened to act if governors refused to, the "lion's share" of Insurrection Act cases have involved presidents acting with the consent of state governors.

"It's generally construed to authorize the president, clearly upon the request of governors, to use the military to enforce state law," he said.

The mobilization of U.S. troops and the federalization of the National Guard "is a shared responsibility and needs to be done in cooperation between state, local, and federal authorities," said Dr. Meryl Chertoff, executive director of the Georgetown Project on State and Local Government Policy and Law.

"A unilateral effort," she said of Trump threatening to act alone, "is only going to be counterproductive."

There are instances in which presidents have mobilized U.S. forces without a governor's consent. President Eisenhower sent in the 101st Airborne Division to Little Rock, Arkansas, in 1957 to protect black students who were integrating into previously-all-white Central High School. Presidents Kennedy and Johnson subsequently invoked the law in the 1960s to enforce civil rights laws.

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University of Texas law professor Steve Vladeck wrote for that the president has "sweeping power" under the law "to use the military for domestic law enforcement" and does not need the consent of state governors to do so. Furthermore, the president can make "the factual determination that the military is necessary," Vladeck said. 

Perez agreed that it is "unlikely" a court would overrule a sitting president on whether his factual determination of the need for federal forces was erroneous.

The law was last invoked by President George H.W. Bush during the 1992 riots in Los Angeles over the acquittal of police officers for assault in the beating of Rodney King. Then-governor of California Pete Wilson asked for federal troops to help quell the riots.

President George W. Bush considered sending federal troops to New Orleans to quell riots after Hurricane Katrina, but decided against it after the Louisiana governor said he did not want the military deployed.

Congress later amended the statute in the 2007 National Defense Authorization Act to liberalize the authority of the president to act without the consent of governors.

However, in the face of backlash by state governors, Congress subsequently withdrew that expansion of executive power, "indicating a congressional unwillingness to broaden the authority of the president to act unilaterally," Chertoff said. 

Trump, she said, "hasn't given a good justification" for using the law, especially since he has not yet issued a proclamation but rather has simply used a "bunch of threats."

If U.S. troops are mobilized, or the National Guard is federalized, the action is also constrained by the Posse Comitatus Act of 1878, which says the troops must be used "in cooperation with police for peaceful purposes," Chertoff said.

It is not to be equated with "martial law," a term that "doesn't really exist in U.S. jurisprudence," Perez said. The closest legal comparison might be the suspension of habeas corpus, which President Lincoln employed without congressional authorization during the Civil War.