You have the order reversed. War has already been declared -- by them. The rational thing to do would be to take care of them, and when the dust settles, take care of the paperwork as a formality, if anyone really cares about it.
You don't need to declare war if the other side has already begun one against you. You do need to wage war, however.
I suspect Israel is approaching the point at which it will simply get up, dust itself off, and take care of things. Not so much out of anger, but out of simple math. Look at the rate of increase of enemy action, the timetables for enemy acquisition of WMD, and so forth, and then, act before it is too late to mount a decisive action.
I would not have too much of a smug, cocky look on my face if I was a denizen of "the arab street" right about now.
my reply is this
how long and when do i get the popcorn? :)