I like the idea of your declaration, but I think the devil is in the details:

• Most have decided that Iraq was sold on misinformation.  Who will decide what a "significant direct attack on an American city or military forces" is?  For example, would the Cole bombing qualify?  
• Could this declaration lead to Administration operatives attacking Americans to instagate a war?
• And who would we trust to tell us who was behind a terrorist act?  I certainly wouldn't trust the CIA with the time of day.