Since taking office, President Donald Trump has sparked controversy over and over with his use of the National Guard - primarily state-based troops that typically respond to issues like natural disasters or large protests. But Trump has sought to use them very differently: deployments to major US cities where residents protest his political agenda, particularly immigration policy.

Trump argues the National Guard is necessary to quell violence in Democratic-controlled cities, support his deportation initiatives, and crack down on crime. But some former military officials, several Democratic governors, and federal judges have expressed concern that this is a major overreach and risks politicising the military.

The latest clash occurred in early October when Trump tried to send National Guard troops to Portland, Oregon, where protesters rallied outside a US Immigration and Customs Enforcement (ICE) building to protest its immigration enforcement tactics. A judge has temporarily blocked the move and the case is ongoing.

All 50 US states, the District of Columbia, and the territories of Guam, Puerto Rico, and the US Virgin Islands have their own contingent of National Guard troops. Those troops also can be deployed abroad, and some units specialise in fighting wildfires or securing the US border.

A little-known provision of US military law governs the president's authority to deploy the National Guard on his own. But up until now, presidents rarely used it. Trump invoked this law to federalise National Guard troops to support his ICE missions.

Trump's attempts to circumvent traditional processes for deploying the National Guard have faced challenges. For instance, he took control of the California National Guard to address protests against immigration raids in Los Angeles, prompting legal challenges.

As protests occurred in Portland, Trump sought to utilise the National Guard following demonstrations against his immigration policies, but a federal judge blocked his efforts, highlighting concerns of overreach in deploying military resources against civil demonstrations.