It was terrorism, NOT a war !!
The perpetrators died, they did not invade the US, had no intention to occupy the US, were just a few volunteers.
Of course many americans will never admit they could have been fooled by so few. That is why they call it a war: to reassure themselves.
The problem is that if it's called a war, that's a pretext for the US administration to engage the army, invade a country and stay.
But that also means you could loose it, you'll need to talk to the enemy which the invader, the US, created.