It would not be a good idea to use a Nuclear Weapon in the region, it would defeat the entire purpose of us being there in the first place.
The whole point is to have it reform into a useful, working, safe, democracy; on it's own. By doing so it will show the rest of the Middle East that they can secure and control their own lives if they support a movement in their own country that does the same thing; without the help of the US Military (like we're doing with Iran already). Then the middle-east will reform and when people no longer rely on a god to better their lives, when they use that god to HELP them during hard times; but rely on THEMSELVES to better their OWN life, there is less violence and terrorism.
Using a Nuclear Weapon would just show the region that there is *no* hope of being able to better their own lives, unless it involves the utmost devotion to Wahabism or Radical Islam.
Not to mention, we're talking about less than 1% of the people in Iraq feeling like this is *OK* to do to anyone, let alone American's. So why kill 99% of the country for their mistakes?