Complex sentence: Six of his sons served during the War of 1812, the eldest, his Alexander Macomb, Jr., was a hero at the Battle of Plattsburgh and later was commander in chief of the United States Army.
Split into simpler sentences: Six of his sons served during the War of 1812. His eldest, Alexander Macomb, Jr., was a hero at the Battle of Plattsburgh and later was commander in chief of the United States Army.