Complex sentence: It was then that the Army of Northern Virginia was reorganized into two Army Corps--the First, led by Lt. General James Longstreet, and the Second, commanded by Lt. General Thomas J. "Stonewall" Jackson.
Split into simpler sentences: It was then that the Army of Northern Virginia was reorganized into two Army Corps. The First led by General James Longstreet, the Second by General Thomas J. "Stonewall" Jackson.