President Donald Trump has put a 19th-century emergency law back into the spotlight, warning he may deploy US troops on American streets as protests intensify in Minneapolis, Minnesota.

The threat follows days of unrest triggered by an expanded operation by US Immigration and Customs Enforcement. Anger exploded after an ICE officer shot and killed a protester, Renee Good, while she sat inside her car last week. Federal officers then shot another man in the leg, an incident authorities say involved self-defence.

As demonstrations swelled, Trump accused Minnesota's leadership of losing control. In a post on Truth Social, he said the state's politicians had failed to stop what he called "agitators and insurrectionists" from attacking ICE officers. If they did not act, he warned, he would invoke the Insurrection Act.

The move would allow Trump to send active duty troops into the state and give them sweeping law enforcement powers.

What exactly is the Insurrection Act?

According to BBC, the American Congress passed the Insurrection Act in 1807, giving the president authority to deploy the military inside the United States during extreme unrest.

The law allows a president to act when violence or rebellion makes it impossible to enforce federal law through the courts. Once invoked, troops can suppress unrest, enforce court orders and carry out arrests. The act also covers the National Guard, which governors usually control during emergencies.

The law's language remains deliberately broad. It sets few limits, outlines no clear triggers and leaves the final decision entirely in the president's hands. That ambiguity gives the White House enormous latitude to use military force at home.

Why is Trump threatening to use it now?

Trump has framed immigration enforcement as a national emergency since returning to office. During his election campaign, he pledged to crush illegal immigration and argued that existing laws and courts stood in the way.

On his first day back in the White House, Trump ordered officials to examine whether the Insurrection Act could help secure the southern border. His administration then launched aggressive measures, including nationwide deportation operations and the transfer of alleged Venezuelan gang members to a prison in El Salvador. Courts have since challenged several of those actions.

Trump has repeatedly raised the act as a weapon against domestic unrest. Last year, he threatened to invoke it after a judge blocked his plan to send National Guard troops to Portland, Oregon. At the time, he said he would act if violence continued and local leaders refused to cooperate.

Minneapolis has now become the latest flashpoint. After weeks of escalating tension and a heavy ICE presence, Trump has once again turned to the Insurrection Act as his ultimate threat.

How has the US used it before?

Presidents have invoked the Insurrection Act sparingly, often during moments of national crisis.

Abraham Lincoln relied on it during the Civil War as southern states rebelled. President Ulysses S Grant later used it to break violent campaigns by the Ku Klux Klan during Reconstruction.

In 1957, President Dwight D Eisenhower sent US Army troops to Little Rock, Arkansas, under the act to enforce school desegregation after the state's governor defied a federal court order.

The last time the act was invoked was in 1992. As Los Angeles burned after the acquittal of police officers in the beating of Rodney King, President George H W Bush deployed Marines, Army units and National Guard troops to restore order.

Are there limitations to the use of this act?

The US has long resisted the use of soldiers as domestic police. The Posse Comitatus Act of 1878 restricts military involvement in law enforcement, but the Insurrection Act overrides those limits.

States usually deploy their own National Guard units during unrest. However, Trump has expanded federal power by declaring national emergencies, unlocking authorities normally reserved for wartime or disaster scenarios.

Since returning to office, he has used those powers to impose tariffs and deploy federal officers, National Guard units and active duty troops to cities including Washington DC, Los Angeles and Memphis.

In March, Trump revived another rarely used law, the Alien Enemies Act of 1798, to deport migrants he labelled gang members. Courts moved quickly to limit its reach, with the Supreme Court placing temporary restrictions on its use.

If Trump indeed decides to trigger the Insurrection Act, legal challenges will almost certainly follow. Whether the courts can rein him in, and how far a president can go once troops take to the streets, remains an open and deeply contentious question.

Donal Trump / US Immigration and Customs Enforcement (ICE) / Minneapolis / protest / US Insurrection Act / Explainer