They advised Yuan Shao to start a campaign against Cao Cao when the latter was still building up his forces .
Yuan Shao ignored their advice as Cao Cao was still nominally Yuan Shao 's Cao Cao .
They advised Yuan Shao to start a campaign against Cao Cao when the latter was still building up his forces , but Yuan Shao ignored their advice as Cao Cao was still nominally his ally .