President Donald Trump has faced various legal challenges regarding his use of the Alien Enemies Act, including a challenge heard by a federal appeals court on Monday.

The U.S. Court of Appeals for the 5th Circuit is considering whether the president properly invoked the Alien Enemies Act to deport alleged gang members.

The case is likely to be the first to reach the Supreme Court on the substantive issue of the president’s invocation of a rarely used wartime law.