Complex sentence: He joined the navy during the Seven Years'War, being made a lieutenant in 1756 commanding the fireship "Pluto", a ship which was attached in 1757 to Sir Edward Hawke's fleet for the Rochefort expedition.
Split into simpler sentences: He joined the navy during the Seven Years'War. He was made a lieutenant in 1756 commanding the fireship "Pluto", and as such was present on Sir Edward Hawke's 1757 Rochefort expedition.