President Donald Trump has issued a warning that he could use a centuries-old act to send military personnel into specific US states.

Trump cited the Insurrection Act, which is a law that allows the president to deploy US military forces on home soil, which comes during an ongoing showdown with Democrat-led cities.

This wouldn’t be the first time the president has used his power to deploy assistance as, earlier this year, he deployed 800 members of the National Guard into Washington DC for what he described as a ‘crime emergency’.

Trump has also sent troops into other places including Chicago and Los Angeles to tackle other issues such as protests.

There has been an ongoing shutdown of the US government since the start of October after Republicans and Democrats couldn’t reach an agreement over a new spending bill.

So, what is the Insurrection Act and what would enforcing it mean for Americans?

The act was first put into law back in 1807 and allows the president to utilize federal military troops or federalize National Guard troops in specific circumstances.

It may be used to manage uncontrollable protests or other civil disturbance situations, such as an armed rebellion.

As per Sky News, the statute states that it can be invoked when there are ‘unlawful obstructions, combinations or assemblages or rebellion’ against the authority of the US government.

President Thomas Jefferson signed it into law in 1807, and as you can imagine, it's very rarely used.

The Guardian reports that it was used frequently during the civil rights movement in the 1960s to protect activists and students who were desegregating schools.

George HW Bush also used it to respond to riots in Los Angeles in 1992.

Why does Trump want to use the Insurrection Act?

Trump is attempting to send National Guard troops to Democrat-led cities to address what he is calling ‘out of control crime’, but has come into some legal problems.

On Monday, a federal judge temporarily blocked the Trump administration from deploying troops to Portland, Oregon, according to the BBC.

After the president deployed the guards in response to the Californian protests against immigration raids, the state filed two cases against him, with one being ruled in his favour, while the other found that the deployment had violated the Posse Comitatus Act, which generally prohibits military personnel from being used for domestic law enforcement purposes.

Speaking about the act from the Oval Office on Monday, Trump told reporters he would use it ‘if it was necessary’ but so far it ‘hasn't been’.

He said: “But we have an Insurrection Act for a reason. If I had to enact it, I'd do that - if people were being killed ... and courts were holding us up, or governors or mayors were holding us up, sure, I do that. I mean, I want to make sure that people aren’t killed. We have to make sure that our cities are safe.”