I've often felt that the horror of what happened that day has been a huge contributor to the way America has fallen further into divisive rhetoric and division between its constituent societies in the subsequent years.
Pre 9/11 I think the USA felt like it was invulnerable from attack, but this terrorist atrocity punctured that misconception. The fear response tends to send people running (sometimes harmfully) towards the familiar, the accepted, and the comfortable.
I know we like to look to social media as the cause of division, but I think its only accelerated a state of being that was started on September 11th.
Not just an issue in America either. The entire western hegemony was rocked to its core by 9/11 (New York, after all, is the most multicultural and international of all US cities). We all felt the after effects, and continue to do so.
Mods - delete if this is too political. but I think it's important we recognise that the effects of this atrocity are still very much felt to this day. It really did change the world. And the best way we can honour the people who died is to not forget that, and try to do better in their name.