Complex sentence: During that campaign he successfully cleared Leesburg, Virginia of Union forces at the Battle of Mile Hill so that the army could cross the Potomac from there and led his troops in a key defensive position protecting Crampton's Gap at the Battle of South Mountain.
Split into simpler sentences: During that campaign he successfully cleared Leesburg, Virginia of Union forces at the Battle of Mile Hill so that the army could cross the Potomac River from there. He led his troops in a key defensive position protecting Crampton's Gap at the Battle of South Mountain.