Complex sentence: 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.
Split into simpler sentences: Sheridan took command of all forces in West Virginia, western Maryland and the Middle Military Division responsible for operations around the Shenandoah Valley. His field army was called the Army of the Shenandoah.