Wouldn't a decleration of war remove the element of suprise?
Psst! Hey, we're going to declare war on you, just so you know. So you'll be ready.
Maximum damage to enemy forces is best achieved when you catch them unawares. Feels like a "Well, duh!" moment.
No disrespect meant.
Didn't they see the incoming aircraft on radar, but they thought it was B-17's or something?