Rules Of War

With all the press and celebration about the assasination of Abu Musab al-Zarqawi, with the photos of his dead body, and the detailed speculations of just how long he lived following the attack (52 minutes), it's got me sincerely wondering if I've misunderstood the justifications and objectives of war. I thought war was about principles and ideals - democracy, freedom, liberty - and that justice belonged in the court systems. How is it OK that our government is targeting individuals, as if to mete out justice? Does that mean that our own President can be legitimately targeted for assasination - meaning that if our enemies succeeded, we could grieve, but would have to accept this as a justifiable and legitimate consequence of being at war? I would sincerely like to hear an explanation of the United States government's rules of war that justify this celebration of assasination.