Complex sentence: After fighting at the First Battle of Manassas, Walton and his command were assigned to James Longstreet's forces and served on the Peninsula, at Second Manassas, and Antietam.
Split into simpler sentences: After fighting at the First Battle of Manassas, Walton and his command were assigned to James Longstreet's forces. These batteries served in the Peninsula Campaign, at Second Manassas, and Antietam.