What is the Insurrection Act, and can Trump use it?

President has threatened to invoke 19th-century law as a means to control mobilization of national guard troops

Donald Trump has once again threatened to invoke the Insurrection Act, a law authorizing the president to deploy military forces on US soil, as a means to control the mobilization of the national guard as courts and governors in Democratic-led cities continue to stymie his attempts.

But can he do that, and what does it mean? Here's what to know about the centuries-old law.