Complex sentence: The primary mission of a navy was to secure the command of the sea, which would permit the maintenance of sea communications for one's own ships while denying their use to the enemy and, if necessary, closely supervise neutral trade.
Split into simpler sentences: The primary mission of a navy was to secure the command of the sea. That not only permitted the maintenance of sea communications for one's own ships while denying their use to the enemy but also, if necessary, provided the means for close supervision of neutral trade.