Complex sentence: Napier next took "Euryalus" across the Atlantic to serve in the War of 1812, including the expedition up the Potomac (August--September 1814), in which he was second in command of the squadron under James Alexander Gordon.
Split into simpler sentences: Napier next took "Euryalus" across the Atlantic to serve in the War of 1812. Next, Napier and "Euryalus" participated in the expedition up the Potomac (August--September 1814), in which he was second in command of the squadron under James Alexander Gordon.