The UCC was developed by UNESCO as an alternative to the Berne Convention for those states which disagreed with aspects of the Berne Convention, but still wished to participate in some form of multilateral copyright protection. These states included developing countries and the Soviet Union, which thought that the strong copyright protections granted by the Berne Convention overly benefited Western developed copyright-exporting nations, and the United States and most of Latin America. The United States and Latin America were already members of a Pan-American copyright convention, which was weaker than the Berne Convention. The Berne Convention states also became party to the UCC, so that their copyrights would exist in non-Berne convention states.
The United States only provided copyright protection for a fixed, renewable term, and required that in order for a work to be copyrighted it must contain a copyright notice and be registered at the Copyright Office. The Berne Convention, on the other hand, provided for copyright protection for a single term based on the life of the author, and did not require registration or the inclusion of a copyright notice for copyright to exist. Thus the United States would have to make several major modifications to its copyright law in order to become a party to it. At the time the United States was unwilling to do so. The UCC thus permits those states which had a U.S.-style system of protection for fixed terms at the time of signature to retain them. (Eventually the United States became willing to participate in the Berne convention, and change its national copyright law as required, and in 1988 it became a party to the Berne Convention.)
Berne Convention states were concerned that the existence of the UCC would encourage parties to the Berne Convention to leave that convention and adopt the UCC instead. So the UCC included a clause stating that parties which were also Berne Convention parties need not apply the provisions of the Convention to any former Berne Convention state which denounced the Berne Convention after 1951. Thus any state which adopts the Berne Convention is penalised if it then decides to denounce it and use the UCC protections instead, since its copyrights would may no longer exist in Berne Convention states.
Most of the states parties to the UCC are now also parties to the Berne Convention, so the UCC is largely irrelevant today.