President Donald Trump deployed 2,000 California National Guard troops to Los Angeles to respond to immigration protests, over the objections of Gov. Gavin Newsom.

It’s not the first time Trump has activated the National Guard to quell protests. In 2020, he asked governors of several states to send troops to Washington, D.C. to respond to demonstrations that arose after George Floyd was killed by Minneapolis police officers. Many of the governors he asked agreed, sending troops to the federal district. The governors that refused the request were allowed to do so, keeping their troops on home soil.

This time, however, Trump is acting in opposition to Newsom, who under normal circumstances would retain control and command of California’s National Guard. While Trump said that federalizing the troops was necessary to “address the lawlessness” in California, the Democratic governor said the move was “purposely inflammatory and will only escalate tensions.”

Here are some things to know about when and how the president can deploy troops on U.S. soil.

The laws are a bit vague: Generally, federal military forces are not allowed to carry out civilian law enforcement duties against U.S. citizens except in times of emergency.

An 18th-century wartime law called the Insurrection Act is the main legal mechanism a president can use to activate the military or National Guard during times of rebellion or unrest. But Trump didn’t invoke that act on Saturday.

Instead, he relied on a similar federal law that allows the president to federalize National Guard troops under certain circumstances.

The National Guard is a hybrid entity that serves both state and federal interests. Often it operates under state command and control, using state funding. Sometimes National Guard troops will be assigned by their state to serve federal missions, remaining under state command but using federal funding.

The law cited by Trump’s proclamation places National Guard troops under federal command. The law says that can be done under three circumstances: When the U.S. is invaded or in danger of invasion; when there is a rebellion or danger of rebellion against the authority of the U.S. government, or when the president is unable to “execute the laws of the United States,” with regular forces.

But the law also says that orders for those purposes “shall be issued through the governors of the States.” It’s not immediately clear if the president can activate National Guard troops without the order of that state’s governor.