President Trump is testing a 147-year-old law that says the military can't typically engage in standard police work. He federalized the District of Columbia police department this summer and sent National Guard members to the capital, as well as to Los Angeles in June. (A 30-day takeover of D.C. policing ended last week, though the Army will keep guard units there until Nov. 30. City leaders challenge the action in court.) The president also spoke last week of sending federalized National Guard troops to Memphis, Tenn., where Mayor Lee Harris isn't welcoming. "To have individuals with military fatigues, semiautomatic weapons and armored vehicles patrolling our streets is way too far, anti-democratic and anti-American," he said.

Earlier, Trump listed Chicago, Boston, Baltimore, New York, New Orleans, New York City and Portland, Ore., as potential sites for a military crime-fighting presence. Separately, hundreds of Immigration and Customs Enforcement (ICE) agents surged into Boston and Chicago last week. State and local officials weren't notified in advance, they say. Critics say the deployments are intended to punish Democratic-led cities and spread fear in immigrant communities. "There is no emergency that warrants deployment of troops," said Illinois Gov. J.B. Pritzker in response to that possible move.

While "crime emergency" responses in the capital are allowed under its special status as a federal district, sending soldiers to a state without the governor's request would likely spark a legal battle like one in Los Angeles this summer. A district judge there ruled this month that the administration broke federal law by using thousands of National Guard members and a smaller number of Marines in response to protests over immigration raids.

Professor Luke William Hunt of the University if Alabama, a policing scholar and former FBI special agent, posts in a recent commentary: "The plan to continue using National Guard troops to reduce crime in cities such as Chicago and Baltimore violates the legal prohibition against domestic military law enforcement." Others experts say presidents deserve near-total legal leeway for military use, even under the Posse Comitatus Act signed in 1878 by President Rutherford B. Hayes. Andrew Stoltman, a Chicago attorney and law professor at Northwestern University, expects the Supreme Court to eventually allow Trump's unconventional use of the National Guard. The court has sided with him in temporary rulings this year.

Trump says: "Chicago about to find out why it's called the Department of WAR." – Sept. 6 social media post

Governor says: "Illinois won't be intimidated by a wannabe dictator." – J.B. Pritzker

Mayor says: "Chicago doesn't want to see reckless, unconstitutional, militarized immigration enforcement in our city." – Brandon Johnson