Complex sentence: This banned so-called "paper" blockades--blockades that were declared to the blockaded nation, but were not actively enforced, allowing the blockading party to seize the cargo of neutral states trading with blockaded harbors.
Split into simpler sentences: This banned so-called "paper" blockades--blockades that were declared to the blockaded nation, but were not actively enforced. Such a blockade did, however, allow the blockading party to seize the cargo of neutral states trading with blockaded harbors.