1941 Wartime Measure (An act to authorize the refusal of visas to aliens whose admission into the United States would endanger the public safety)
S. 913; Pub.L. 77-113; 55 Stat. 252.
77th Congress; June 20, 1941.
You can find the full text of this law as a PDF here.
This Act held that whenever any American diplomatic or consular officer knew or had a reason to believe that any alien sought to enter the United States for the purpose of engaging in activities which will endanger society they could refuse any visa or temporary work visa, thereby denying admission to the US.
(Summary by Juan C. Gallo)