President Trump is invoking one of the most chilling lines from a classic 1979 movie in his latest warning of plans to deploy the National Guard to fight crime in Chicago.

“‘I love the smell of deportations in the morning…’ Mr. Trump writes in a nod to the Vietnam War movie “Apocalypse Now.” He continues: “Chicago about to find out why it’s called the Department of WAR.”

The accompanying image depicts Mr. Trump as a character from Francis Ford Coppola’s Oscar-winning movie, in sunglasses and a U.S. Calvary Stetson hat. Behind him are flames and military helicopters seen against the Chicago skyline.

The text is a takeoff on Robert Duvall’s memorable line from the movie, where he says, “I love the smell of napalm in the morning.” Troops commanded by Mr. Duvall’s character, Lieutenant Colonel Bill Kilgore, had just used napalm on a village.

Mr. Trump has said repeatedly that he plans to send troops to Chicago, following on what he sees as a highly successful deployment of federal forces to Washington, D.C.

The mayor of Chicago has said National Guard troops will not be welcomed and the governor of Illinois, JB Pritzker, says the state is prepared to take Mr. Trump to court if the military is sent into that city.

Mr. Trump has pointed to Chicago’s murder rate as justification for the federal intervention, and some Democratic leaders are open to the idea. Alderman Raymond Lopez has criticized Mayor Brandon Johnson on crime and says the city should consider welcoming Mr. Trump’s offer.

Mr. Trump has been pleased with the results in Washington, D.C., where crime has been substantially reduced, and now wants to replicate that success in cities across the country. He has said that New York could be targeted for a deployment soon.

Maryland officials are hoping to stave off Mr. Trump’s threat to deploy National Guard troops to Baltimore. Governor Wes Moore, along with Baltimore’s mayor, Brandon Scott, announced late Friday that the Maryland State Police and other state law enforcement officers would be surged to the city to assist the Baltimore Police Department.

While popular with some residents of high-crime neighborhoods, the legality of Mr. Trump’s deployments is uncertain. A federal judge ruled this week that the administration violated the law when it responded to anti-ICE protests in June by federalizing the National Guard and deploying Marines at Los Angeles.

Officials said the deployment of 4,000 National Guard members and 700 Marines was intended to help protect federal property and personnel from protesters as immigration officials carried out deportation operations.

Critics argued that the deployment violated the Posse Comitatus Act of 1878, which prohibits military personnel from engaging in domestic law enforcement activities with certain exceptions. The Trump administration maintained the troops were not engaging in law enforcement activities but were simply protecting federal property.

The Trump administration is expected to appeal the ruling.