Trump then escalated the widening clash with Democratic-led states and cities over the domestic use of military forces, threatening to invoke the Insurrection Act as a means to circumvent court restrictions on deploying troops where they are unwanted by local officials.
Illinois had sued in response to Pentagon chief Pete Hegseth's orders over the weekend to bring 300 Illinois National Guard members under federal control and then to mobilise another 400 Texas National Guard members for deployment to Chicago.
While Illinois' request for a temporary restraining order plays out, US lawyers told a court hearing on Monday that Texas National Guard troops were already in transit to Illinois. Trump then issued another memorandum calling up the Illinois National Guard, reinforcing Hegseth's previous order.
US District Judge April Perry allowed the federal government to continue the deployment in Chicago while it responds to Illinois' suit. She set a deadline of midnight Wednesday for the US to reply.
Shortly after that ruling, Trump told reporters in the Oval Office he might invoke the Insurrection Act of 1792, which would allow troops to directly participate in civilian law enforcement, for which there is little recent precedent.
"I'd do it if it was necessary. So far, it hasn't been necessary. But we have an Insurrection Act for a reason. If I had to enact it, I'd do that," Trump said.
"If people were being killed and courts were holding us up, or governors or mayors were holding us up, sure, I'd do that."
Today, Democratic-led states and cities are pushing back against Trump's attempt to deploy military forces into cities, which the White House says are needed to protect federal government employees from "violent riots" and "lawlessness".
Democratic leaders counter that their cities are being illegally targeted and falsely portrayed as awash in crime.
Trump has expanded the use of the US military in his second term, which has included deploying troops along the US border and ordering them to kill suspected drug traffickers on boats off Venezuela without due process.
National Guard troops are state-based militia forces that answer to their governors except when called into federal service.
Trump has ordered them to Los Angeles, Chicago, Washington DC and Portland, prompting lawsuits from state and local leaders.
Chicago's lawsuit is the fourth legal action opposing Trump's unprecedented use of soldiers to police US cities. Courts have not yet reached a final decision in any of those cases, but judges in California and Oregon have made initial rulings that Trump likely overstepped his authority.
The Illinois lawsuit alleges the Republican president is deploying the military to Illinois based on a "flimsy pretext" that an ICE facility in a suburb of Chicago needs protection from protesters.
Illinois Governor JB Pritzker, in a press conference, accused Trump of unnecessarily escalating tensions by attempting to add National Guard troops to heavily armed federal police from Immigration and Customs Enforcement and other agencies already operating in Chicago.
Pritzker said those officers have fired tear gas and rubber bullets on peaceful protesters, with US citizens, including children, being "traumatised and detained."
"Donald Trump is using our service members as political props and as pawns in his illegal effort to militarise our nation's cities," Pritzker said.
"Donald Trump's deranged depiction of Chicago as a hellhole, a war zone and the worst and most dangerous city in the world was just complete BS," Pritzker said.
