Complex sentence: In 1648 the lord of the Isle of Man, James Stanley, 7th Earl of Derby, appointed Christian his receiver-general; and when in 1651 the earl crossed to England to fight for Charles II he left him in command of the Island militia.
Split into simpler sentences: In 1648 the Lord of Mann, James Stanley, 7th Earl of Derby, appointed Christian as Receiver General. In 1651 the Earl went to England to fight for Charles II and Christian was left in command of the Island militia.