All the King's horses and all the King's men can't put Iraq back together again.
The Iraqi's have to do that for themselves. Note the recent car bombing of the Shiite cleric.
They're not ready to put Iraq back together again yet. Won't make any difference who's troops or how many are there until IRAQIS decide they've had enough.
We (the US) are going to have to stay and do what we can to preserve basic order and services until (if?) the Iraqis decide they want the rule of law and order. If they prefer the 'religious gang warfare' situation, won't be much we or anyone can do until they've had enough of shedding their own blood.
As for attacks against US troops, you've got to expect that. You've got the old regime's holdout/hardliners that still want to fight and you've got the Isalmic jihadis coming in from other countries to take a potshot at the Great Satan's troops.
Better to have them taking on our military in Iraq than taking on our civilians in the US.
But hey, the UN might be THE SOLUTION! Look how well UN intervention in the Israeli/Arab/Palestinian situation quickly resolved the problem and led to a long-lasting peace.
(Not a knock on the UN, actually. Just points out that until folks would rather live than die, there's nothing much anyone can do to help them.)