Complex sentence: In 1862, he was transferred to the field as chief engineer of the Army of the Potomac and, in 1863 during the Siege of Vicksburg, as the chief engineer of the Army of the Tennessee.
Split into simpler sentences: In 1862, he was transferred to the field, eventually becoming chief engineer of the Army of the Potomac. In 1863 during the Siege of Vicksburg, he served as the chief engineer of the Army of the Tennessee.