Originally Posted by The Joker
I don't buy that at all. I don't see the Mayor, Dent and Gordon passing up the chance to net the Joker or give him the chance to kill more people. If they thought they had a chance to catch him then they would have been there ready and waiting with every Cop they had. Even call in the National Guard if they had to. Joker said he would do it at midnight, so they had a time frame to be get prepared.
Napier & Grissom and that whole "crime syndicate" were all in league with one another. The police force didn't have the power to topple them. That's why they brought in Harvey Dent in the first place... things were just starting to turn (hopefully) and then with Joker's emergence? They were left a bit helpless. At least I think that's what you're supposed to gather from their relative impotency.
Maybe the National Guard doesn't exist in the DCU? Maybe Gordon and the powers that be were going to let Batman deal with the Joker, and they cleared the streets accordingly? Maybe I'm making excuses for plot holes, but you have to resign to the fact that they are there, and while there are ways of explaining them, the world in that film is not the same world we live in. It's a movie about a fictional place, so give it a little leeway eh?