Complex sentence: During this period, Confederate General James Longstreet unsuccessfully besieged the town with 20,000 men between April 11,1863, to May 4,1863--finally being ordered to disengage by General Robert E. Lee to join the Army of Northern Virginia at Fredericksburg.
Split into simpler sentences: During this period, Confederate General James Longstreet unsuccessfully besieged the town with 20,000 men between April 11 and May 4,1863. He was ordered to disengage by General Robert E. Lee to join the Army of Northern Virginia at Fredericksburg.