Bush has little or no capital left to start anything with Iran. They know that and that is why they are doing it. They never would have done this in 2003 or even 2004. But they smell blood in the water and are acting in their own best interest. They know it's just a matter of time before Bush is gone and the next president (R or D) will not want to spend all their political capital on waging this unpopular war and will look for the earliest exit. All they have to do is not poke the bear too egregiously but keep up the pressure to make this an unpopular war.
They never would have done this in 2003 or even 2004.
Of course he would have to stop babying the Iraqs and turn our troops loose first... but that would go against his kinder gentler compassionate KGC warfighting.
That's exactly what's going to get mullahs in trouble. They are getting more brazen and transparent. Due to "no WMDs found in Iraq" and previous failures of CIA (real and imagined) and feeble attempts by Bush and his administration to properly explain what we have been and still are doing Iraq, people (and particularly, Congress) may not be too supportive of another military action against Iran's nuclear program and its potential for WMDs and undefined timeline.
However, the documented cases of acts of war by Iran against Iraqis and coalition forces in Iraq, and failure to stop them after several meetings and warnings face-to-face about the subject, will give us undeniable proof and casus belli, justification and pretext for military actions against Iran's military and nuclear installations inside Iran (which in turn will likely help provide a regime change).
Quad erat faciendum.