Traditionally copyright protection has been territorial. That is, national law will apply to acts of infringement committed in a particular country, regardless of the national origin of the work infringed. The principal multilateral copyright conventions aim to promote international exchange of works of authorship by mandating the nondiscrimination rule of "national treatment." While promoting the permeability of national boundaries by copyrighted works, this rule also preserves national sovereignty by confining any country's copyright regime to local borders.
There is no such thing as an "international copyright" that automatically protects a work throughout the world although more than 150 countries have ratified a treaty intended to accomplish as many of the benefits of "international copyright" as possible. Generally, if a work is protected in the U.S. it is protected in most countries because the U.S. adheres to the leading copyright convention, the Berne Convention, which is administered by the World Intellectual Property Organisation (WIPO). The same is true for works published in other countries. They are under the copyright protection in this country if the country in which the work has been created is a signatory of international copyright agreements.
Most foreign publishers of books, journals, and audiovisual materials have distributors or representatives in the US. If you need to use works published in other countries and your use does not fall into fair use guidelines, you should contact these representatives to get permission.