If you could go back in time, how you prevent 9/11

I am sorry but that is wishful thinking They say if you tried to change things the world would be entirely different and so many things would be affected that is best not to speculate. I cannot think of one thing that could have been done to change things and to be honest would not it better to say if we could go back to 1939 how could we have avoided World War II ...and so many other incidents I am not minimizing the tragedy and horrendous Sept 11th but there are a dozens of tragedies that have unfortunately occurred.

Cheerio: I had better be shoveling off

First of all, hindsight is 20/20. It's easy to think "what if?" But I also think it's a waste of energy to try and think what one person could have done. Since it was a planned effort of a group of people, I don't think I alone could have stopped it.

And instead of thinking about ways to change the past (since you can't) why not think about ways to make the world a safer place for everyone going forward? Ways to unite us so that events like 9/11 don't happen again?