When the U.S. Department of Justice on Monday released a federal grand jury's indictment of former Trump campaign manager Paul Manafort, as part of Special Counsel Robert S. Mueller's probe into suspected Russian attempts to interfere in the 2016 U.S. presidential election, the charges spoke to a long-running worry among U.S. lawmakers.

Among the 12 charges, Manafort is accused of acting as a "unregistered agent of a foreign principal" and issuing "false and misleading FARA statements," by not properly disclosing the nature of the consulting he did for a Kremlin-linked Ukrainian political party. Failure to do so would be a violation of the Foreign Agents Registration Act (FARA), a registration requirement that makes foreign agents tell the DOJ whom they're working for, what they're doing, and how much they're getting paid for the gig. 

Fear of foreign interference in U.S. affairs is about as old as the U.S., but this 1938 law is a response to a specific fear: Americans in the U.S. working for the Nazis.