A law on the books from 1798, largely thought to be unconstitutional, but that has not been challenged in SCOTUS yet. It allows the government to "During war time, Arrest, relocate, or deport, any male over the age of 14 who 'hailed from' a foreign enemy country" (in WWII we struck the part limiting it to males) It was the law used create internment camps in the US. The law would also apply to green card holders and legal US residents. It's supposed to be used in the event a foreign government or nation had threatened the US and had sent agents and militias to the US ahead of time. Instead it's been used for the darkest moments in US history. And with Trump's administration recently arresting a protesting greencard holder (whom I deeply disagree with many of the statements of, but that still has free speech protections) - it is not hard to see how this will be a horrible and heinous thing to invoke.