The U.S. had joined the Berne Convention on March 1, 1989, when its Berne Convention Implementation Act of 1988 entered in force. Article 18 of the Berne Convention specified that the treaty covered all works that were still copyrighted in their source country and that had not entered the public domain in the country where copyright was claimed due to the expiration of a previously granted copyright there. Consequently, the U.S. would have had to grant copyright on foreign works that were never copyrighted before in the U.S. But the United States denied this retroactivity of the Berne Convention and applied the rules of the treaty only to works first published after March 1, 1989. Earlier foreign works that were not covered by other treaties and that had until then not been subject to copyright in the U.S. remained uncopyrighted in the United States.