Complex sentence: Philip Sheridan, the cavalry commander of the Army of the Potomac, who was given command of all forces in West Virginia, western Maryland, and the Shenandoah Valley, the Middle Military Division; his field army was called the Army of the Shenandoah.
Split into simpler sentences: Philip Sheridan, the cavalry commander of the Army of the Potomac. Sheridan took command of all forces in West Virginia, western Maryland, and the Shenandoah Valley, the Middle Military Division; his field army was called the Army of the Shenandoah.