Complex sentence: When Palpatine was captured by Count Dooku and General Grievous the Battle of Coruscant had started and it was up to Obi-Wan Kenobi and Anakin Skywalker to rescue Palpatine.
Split into simpler sentences: When Palpatine was captured by Count Dooku and General Grievous. The Battle of Coruscant had started and it was up to Obi-Wan Kenobi and Anakin Skywalker to rescue Palpatine.