if they were capable of doing that wouldn't it have been easier to kill them when they were in Iraq?
It's not like NYC isn't a large target all by itself
Having a parade is not going to make them capable (not saying that you're suggesting this) of carrying out an attack
Its not about killing of the people... its about the psychological and political impact of killing those people on our turf.
Same idea with bombing trains/etc.. its not about the people you kill, its about the impact it has on the public.
By definition Terrorism is about changing instilling fear and therefore people changing the way they live because of it, in hopes that the fear and the forced change will cause whoever is the target to do what they want.