Complex sentence: They tried getting the Emperor back but lost again when Cao Cao came to the battle on a secret imperial decree which led to Cao Cao defeating the White Wave Rebels and taking total control over the Emperor.
Split into simpler sentences: They tried getting the Emperor back but lost again when Cao Cao came to the battle on a secret imperial decree. Which led to Cao Cao defeating the White Wave Rebels and taking total control over the Emperor.