Complex sentence: A second relief attempt was made at Tarifa in 1811; however, despite defeating a detached French force of 15,000-20,000 under Marshal Victor at the Battle of Barrosa, the siege was not lifted.
Split into simpler sentences: A second relief attempt was made at Tarifa in 1811. However, despite defeating a detached French force of 15,000-20,000 under Marshal Victor at the Battle of Barrosa, the siege was not lifted.