Question

What legislation allowed the president to arrest foreigners who were believed to be a threat to the United States

Answer 1

It was called the Alien Friends Act and an alien could be deported if he was deemed a threat to the United States peace and/or safety. There was also an Alien Enemies Act that permitted the president to imprison or deport any male that was at least 14 years old and a citizen of a nation the US was at war with.