“I seem to recall from my history lessons in high school that there was a quaint old document created called a “Declaration of War”, passed by the Congress on December 8, 1941. After that declaration, any member of Japan’s military was a legitimate target for attack.”
All a Declaration of War accomplishes it the notification of another power that a state of war exists. A state of war existed the moment the first bomb was dropped on Pearl Harbor and from that moment on all members of Japan’s military became legitimate targets for attack. The Declaration of War was a mere formality.
By the same token, when the first plane crashed into the WTC, a state of war existed between the US and Al-Qaeda and from that moment all members of Al-Qaeda became legitimate targets for attack.
Absent a vote from Congress a state of war would not be authorized under our Constitution.
While I wouldn't disagree that from the moment the first plane crashed into the WTC any killing of any Al Queda member would be perfectly justified - our otherwise Imperial Presidency cannot conduct war unilaterally without authorization from Congress (except apparently in Libya).