Nevermind, of course, that the World Trade Center was owned by the joint port authorities of the States of NY and NJ, not the "Nation".
The answer to your first question can be illuminated a bit by considering a different question: Why do seagoing nations use Naval assets (instead of a coast guard auxiliary) to interdict and defeat pirates on the high seas?
I'm just trying to make a point about warlike actions without formal declarations, in any case.
But I don't even have qualms comparing international waters to Afghanistan, because you have to consider that if a given piece of land is completely ungoverned, does it belong to a specific nation at all?
If you say that the lands in Afghanistan from which AQ organized their terrorism was effectively governed by the Taliban and not a failed state, then it's even easier legally, as you have an actual causus belli for military action in response.
[1] http://en.wikipedia.org/wiki/Hijackers_in_the_September_11_a...
If an Englishman became a pirate on the high seas and was killed by a Spanish warship, would Spain have been at war with England?
> Until then the people have spoken by ballot.
Are you so sure of that?