50 U.S.C. § 21
Section 21 · Restraint, regulation, and removal
Current version, with additions and removals from the April 16, 1918 version.
Whenever there is a declared war between the United States and any foreign nation or government, or any invasion or predatory incursion is perpetrated, attempted or threatened against the territory of the United States by any foreign nation or government, and the President makes public proclamation of the event, all natives, citizens, denizens, or subjects of the hostile nation or government, being of the age of fourteen years and upward, who shall be within the United States and not actually naturalized, shall be liable to be apprehended, restrained, secured, and removed as alien enemies. The President is authorized in any such event, by his proclamation thereof, or other public act, to direct the conduct to be observed on the part of the United States, toward the aliens who become so liable; the manner and degree of the restraint to which they shall be subject and in what cases, and upon what security their residence shall be permitted, and to provide for the removal of those who, not being permitted to reside within the United States, refuse or neglect to depart therefrom; and to establish any other regulations which are found necessary in the premises and for the public safety.
The claim of any naturalized American citizen under the provisions of this Act [sections 1 to 6, 7 to 39, and 41 to 44 of this Appendix] shall not be denied on the ground of any presumption of expatriation which has arisen against him, under the second sentence of section 2 of the Act entitled “An Act in reference to the expatriation of citizens and their protection abroad,” approved March 2, 1907, if he shall give satisfactory evidence to the President, or the court, as the case may be, of his uninterrupted loyalty to the United States during his absence, and that he has returned to the United States, or that he, although desiring to return, has been prevented from so returning by circumstances beyond his control.