In hindsight, collective traumas will often be watersheds of history and signify or initiate times of change in the society affected.
Well known collective traumas include: The John F. Kennedy assassination in the United States, the Estonia disaster in Sweden and various others.
In all likelihood, the destruction of the World Trade Center in the September 11, 2001 Terrorist Attack will induce a severe collective trauma in US society, and indeed affect the entire world through primary and secondary reactions.