The President is Commander in Chief, he decides when and if we go to war.
Read your Constitution. Congress, not the President, has the power to declare war.
Now "use of military force" is not quite the same thing. But remember, Bush asked for, and got from Congress, authorization to use military force against those who attacked us on 9-11, and to keep it from happening again as well as separately to attack Iraq.
Absent an attack from Iran, I don't think President Bush is going to attack them without going to Congress first. Could be wrong though, and there's also the fact that the DemonRats don't actually take over Congress until January. Lots can happen in a month and a half.