Complex sentence: On September 10,1861, Johnston was assigned to command the huge area of the Confederacy west of the Allegheny Mountains, except for coastal areas, and became commander of the Confederacy's western armies in the area often called the Western Department or Western Military Department.
Split into simpler sentences: On September 10,1861, Johnston was assigned to command the huge area of the Confederacy west of the Allegheny Mountains, except for coastal areas. He became commander of the Confederacy's western armies in the area often called the Western Department or Western Military Department.