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 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.

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.