For a moment there one might have been thinking he meant atrocious actions with high death tolls against a civilian population, in which case the OP is definitely failing to understand it with his "for something no other country has ever faced", and the "spare yourself embarrassment by not making such witless pronouncements" is spot on.
Terrorism can be brought on by a state as well as by non state actors. Wikipedia defines terrorism as "the systematic use of terror, especially as a means of coercion." Conscription during any of the major U.S. wars in the 20th century fits this definition nicely. Many more people died from these than on 911.
The death toll from the 9/11 attacks far exceeded any other single terrorist attack.