Complex sentence: In May 1818 Heywood declined command of the Canadian Lakes with the rank of commodore; he had become content with shore life, and said he would only accept another appointment in the event of war.
Split into simpler sentences: In May 1818 Heywood declined command of the Canadian Lakes with the rank of commodore. As he had become content with shore life, he said he would only accept another appointment in the event of war.