Originally Posted by RustyCage
He corrupted Harvey. The peace Batman and Gordon built was founded on a lie.
In that way, the Joker did win.
When Bruce is in the pit, he even has a nightmare of Ra's taunting him about that very thing. It's clearly been bothering him to hell.
Not only did he corrupt Harvey, but he corrupted Gordon and Batman into lying to their city which resulted in borderline police state like legislation. He made them revert on their original goal of inspiring hope in the justice system by shoving the problems of corruption in Gotham under the rug. The Joker won in a way he probably didn't even anticipate, and it's the journey of TDKR where they face the repercussions of this, both physically and mentally.