our own war theory and practice as taught by our own military:
http://reserveweb.fitcpac.navy.mil/isrtm/target.htm
A target is a geographic area, complex, installation, or its contents that has military action planned or directed against it.
Economic Targets Economic targets are civilian resources which contribute to the enemy's warmaking capability. Financial Centers: stock exchanges, financial clearinghouses, electronic fund transfer capabilities Production Sites: factories, electricity plants, petroleum, oil and lubricant (POL) plants Lines of Communication: raw materials movement, civilian workforce movement Trained/Skilled Workforce
so, would it have been acceptable (and not a crime) had they attacked the WTC after, say, 7 p.m.?
well, not according to our own military because:
"It is important to validate the target in terms of its accessability (i.e., is it within range of assets), the target area defenses (i.e, what are the potential threats), and its aimpoints (i.e., can the aimpoint be attacked successfully)."