In a rare use of military force on domestic soil, the Trump administration has deployed National Guard troops and active-duty Marines in Los Angeles to respond to protests set off by its immigration crackdown.

President Trump has long mused about using military force on domestic soil to crush violent protests or riots, fight crime and hunt for undocumented migrants — a move that his aides talked him out of during his first term. Between his two presidencies, he said that he would do so without the consent of state governors if he returned to the White House.

The State of California and its governor, Gavin Newsom, filed a lawsuit on Monday night seeking to overturn Mr. Trump’s move, calling it an unnecessary provocation and unlawful.

Here is a closer look.

What did Trump’s order do?

Mr. Trump called up National Guard troops to be put under federal control, issuing an order late Saturday that authorized Defense Secretary Pete Hegseth to use them to protect immigration enforcement agents, buildings and functions from interference by protesters. As justification, the White House cited recent protests against Immigration and Customs Enforcement raids in Los Angeles.

The order called for at least 2,000 National Guard troops to be deployed for at least 60 days. Mr. Trump also authorized Mr. Hegseth to use regular federal troops “as necessary” to augment the work of the federalized National Guard units.

The National Guard consists of military forces in the state, largely part-time troops who have separate, full-time civilian jobs. Normally, each state’s governor controls its Guard, directing it to deal with a disaster or civil disorder. But under certain circumstances, federal law allows the president to take control.