Complex sentence: Between 1803 and 1812, the United States Navy had a policy of basing the naval forces on coastal gunboats, and experimented with a variety of designs, but they were nearly useless in the War of 1812, and went back to being special-purpose vessels.
Split into simpler sentences: Between 1803 and 1812, the United States Navy had a policy of basing its navy on coastal gunboats. It experimented with a variety of designs, but they were nearly useless in the War of 1812, and went back to being special-purpose vessels.