An extraordinary legal showdown took place last weekend over President Donald Trump’s invocation of an 18th century wartime act to deport hundreds of immigrants to a prison in El Salvador.

The president’s invocation of the wartime law enacted in 1798 was quickly blocked by a federal judge. It had only been used three times before in U.S. history.

The Alien Enemies Act of 1798 gives the president wartime powers to deport undocumented immigrants with little to no due process.