Complex sentence: The First Navy Jack, which was directly related to the Gadsden flag, has been in use by the U.S. Navy, and since the terrorist attack on the WTC and Pentagon, on all active naval ships.
Split into simpler sentences: The First Navy Jack, which was directly related to the Gadsden flag, has been in use by the U.S. Navy. In addition, since the terrorist attack on the WTC and Pentagon, on all active naval ships.