You can go back in time and prevent a great catastrophe.
Which one would you prevent?
I am tempted to say killing Karl Marx n Fred Engles before thay caused any trouble,
but I am cognizant that in the absence of the Second and Third World Wars,
a great deal of Research n Development woud not have been funded,
with the result that we 'd be in a technologically much more primative condition.
That is to say that, in the absence of World Wars 2 and 3,
with all of their deaths, suffering, slavery under the commies n nazis,
we 'd probably not have landed on the Moon yet, and
I 'd not have a personal computer nor a High Defintion TV ;(I like
my HDTV and my computer).