9/11 was an attack.

No foreign forces occupied territory of the United States, which would be the criteria for an invasion.

These are all well-established definitions in the legality of war.

Edit- and before we take the pacifist "there should be no legal war" approach, that may be so, but we live in a world where international law delineates just and unjust war, and applies strict definitions to do so, so when discussing conflict happening in the real world and not theoretically, it must be done within the confines of its practical definitions.