Complex sentence: He served the Cause of Parliament by raising a regiment of infantry in June 1643 and then he went on to serve the Commonwealth of England and, in 1656 he became a General at Sea.
Split into simpler sentences: He served the Cause of Parliament by raising a regiment of infantry in June 1643. He continued to serve in the army for the Commonwealth of England and, in 1656 he became a General at Sea.