What is the 1798 law that Trump used to deport migrants?

More than 200 Venezuelans alleged by the White House to be gang members have been deported from the US to a notorious mega-jail in El Salvador.

Out of the 261 people deported, 137 were removed under the Alien Enemies Act, a senior administration official told CBS News, the BBC's US partner.

This broad, centuries-old law was invoked by President Donald Trump. He accused Venezuelan gang Tren de Aragua (TdA) of "perpetrating, attempting, and threatening an invasion or predatory incursion" on US territory.

The move has been criticised by rights groups, and came despite a temporary block issued by a judge. The White House said the judge's order itself was not lawful and was issued after the group was deported.