On his first day back in the White House, President Donald Trump signed an executive order declaring an emergency at the US southern border and directed his top officials to evaluate whether to invoke a rarely used 19th-century law to respond to that emergency.
The Insurrection Act of 1807 allows the president to use active-duty military personnel to perform law-enforcement duties inside the US.
The law has recently come up again, as Trump sends National Guard troops and Marines to the immigration-related protests in downtown Los Angeles, which he describes as violent, over the objections of the mayor and California governor.
"If there's an insurrection, I would certainly invoke it," Trump said on Tuesday. "We'll see."
Here's what to know.
