"'I love the smell of deportations in the morning,'" Trump wrote on his social media site. "Chicago about to find out why it's called the Department of WAR."
The president offered no details beyond the label "Chipocalypse Now," a play on the title of Francis Ford Coppola's dystopian 1979 film set in the Vietnam War, in which a character says: "I love the smell of napalm in the morning."
In response to the post, Illinois governor JB Pritzker, a Democrat, called Trump a "wannabe dictator".
Trump on Friday signed an executive order seeking to rename the Defence Department the Department of War, after months of campaigning to be considered for the Nobel Peace Prize. The renaming requires congressional approval.
The illustration in Trump's post shows him against a backdrop of the Chicago skyline, wearing a hat matching that of the movie's war-loving and amoral Lieutenant Colonel Kilgore, played by Robert Duvall.
Trump's weekend post follows his repeated threats to add Chicago to the list of other Democratic-led cities he's targeted for expanded federal enforcement.
In addition to sending troops to Los Angeles in June, Trump has deployed them since August in Washington, as part of his unprecedented law enforcement takeover of the nation's capital.
He's also suggested that Baltimore and New Orleans could get the same treatment, and on Friday even mentioned federal authorities possibly heading for Portland, Oregon, to "wipe 'em out", meaning protesters.
He could have been mistakenly describing video from demonstrations in that city years ago.
Details about Trump's promised Chicago operation have been sparse, but there's already widespread opposition.
City and state leaders have said they plan to sue the Trump administration. Pritzker, a possible 2028 presidential candidate, is also fiercely opposed to it.
The president "is threatening to go to war with an American city," Pritzker wrote on social media platform X over an image of Trump's post. "This is not a joke. This is not normal.
"Donald Trump isn't a strongman, he's a scared man. Illinois won't be intimidated by a wannabe dictator."
Trump has suggested he has nearly limitless powers when it comes to deploying the National Guard. At times he's even touched on questions about his being a dictator.
"Most people are saying, 'If you call him a dictator, if he stops crime, he can be whatever he wants' - I am not a dictator, by the way," Trump said in August.
"Not that I don't have - I would - the right to do anything I want to do," he added.
"I'm the president of the United States," Trump said. "If I think our country is in danger - and it is in danger in these cities - I can do it."
