Complex sentence: The establishment of such relations, however, requires permission from the U.S. Department of State or Congress itself--most, however, are already set by existent laws or trade agreements established beforehand by the United States that supersede the relation pursued by Puerto Rico.
Split into simpler sentences: The establishment of such relations, however, requires permission from the U.S. Department of State or Congress itself. Most relations, however, are already set by existent laws or trade agreements established beforehand by the United States, and that thus, supersede the relation pursued by Puerto Rico.