No, “terrorism” is no more legal or illegal than any other violent act. It is merely an effective tactic, & often the only tactic, available against an overwhelming force. Only the overpowering side considers such well-targeted strikes as 9/11 as “illegal”, whatever that means.
I, & any fair-minded neutral judge, must consider 9/11 as the most creative military action of all time. To plan & coordinate so many unique details & to aim & successfully carry out the primary assault on The World Trade Center, the most blatant swaggering symbol of corporate sleaze, corruption, & greed, took the most genius military mind ever. To do it with 2 planes within minutes of each other while a 3rd plane struck another equally imperious & destructive institution, the Pentagon, was brilliance beyond compare.
Any stick-in-the-mud militarist with an enormous army can march across a field & slaughter millions, but if the lesser force in such conditions hides behinds trees & boulders or in ditches, is that unfair & warrants to be branded as “terrorism”? No, it’s just that the American colonists used their noodles better than did the tradition bound Brits who marched to a drum into a hail of bullets coming from all sides of an open field with no firing enemy in sight. The Native Americans had burned the asses of the colonists with this tactic & the colonists then turned it on their foes, the regimented…