As the United States Copyright Office explains, there is no single law governing "international copyrights." The laws of the individual countries govern, as do international treaties and agreements.
One provision that may apply if you are a researcher attempting to use information in the United States that originates in a foreign country is Section 104A of the Copyright Act. Section 104A of the Copyright Act restores copyright protection to international works that would have otherwise fallen into the public domain due to a failure to follow United States legal formality requirements (such as, during the years between 1924-68 a failure to register the work with the US copyright office or a failure to renew such a registration).
However, there is an exception to restoratoin in the United States for works that were in the public domain in their home country as of January 1, 1996 (which is why it is important to also understand the term limit for copyright in the international country as well).
If Section 104A does operate to restore protection to an international work (this provision is limited to countries adhering to the Berne Convention, the WIPO Treaty and the like) then the term of protect in the US for the international work becomes 95 years from the date of publication of the work.
Again, this is complicated, so do not hesitate to contact the Copyright Librarian, Sara Benson at email@example.com, with any questions that remain.