Also called the Act of October 6, 1917, it was "An Act to define, regulate, and punish trading with the enemy, and for other purposes." As the United States entered into WWI it became apparent that there were enemies living within the boundaries of the United States, and it became necessary to determine who could be labeled an enemy. The act specifically exempted citizens when it was written: "other than citizens of the united States."
However the Act of March 9, 1933, Section 2, specifically amended that to include: "any person within the United States or any place subject to the jurisdiction thereof."
These provisions only take place when the United States is under a state of emergency, which is why every president has declared an emergency since 1933.
Several Constitutional protections are subject to this state of emergency (or public danger):