1918 Wartime Measure (An act to prevent in time of war departure from or entry into the United States contrary to the public safety)
H.R. 10264; Pub.L. 65-154; 40 Stat. 559.
65th Congress; May 22, 1918.
You can find the full text of this law as a PDF here.
This Act asserted that any alien leaving or entering the country was subject to comply with the rules and regulations prescribed by the president. People who planned to use false documents to enter or depart the U.S. were considered criminals and were subject to deportation and fines. Every U.S. citizen was required to bear a passport if they planned to depart or enter the United States. People, corporations, individuals, associations, etc. living or traveling through U.S. territories and canals were subject to U.S. jurisdiction.
(Summary by Juan C. Gallo)
President Urges Longer Alien Ban - Article from the New York Times, August 25th, 1919.
Enemy Aliens Now Need no Permit to Quit the United States - Article from the New York Times, July 2nd, 1920.