Complex sentence: On September 3, just two days after the Battle of Chantilly, Lee wrote to President Davis that he had decided to cross into Maryland unless the president objected and on that same day began shifting his army north and west from Chantilly towards Leesburg, Virginia.
Split into simpler sentences: On September 3, just two days after the Battle of Chantilly, Lee wrote to President Davis that he had decided to cross into Maryland unless the president objected. On the same day, Lee began shifting his army north and west from Chantilly towards Leesburg, Virginia.