system
Given the following post, respond as if you're commenting on Reddit:
human
[Watchmen] Could Dr Manhattan have prevented 9/11

Or would it have been like with JFK where he saw what happened but didn’t change it because it already happened or like Before Watchmen where he saw two different realities?

What do you think the fallout would have been if he saw it but failed to prevent it?
gpt
He could  easily prevent it, but it probably wouldn't be particularly interesting or notable to him. 

Combustion changed some compounds into other compounds and the universe spins on, nothing to concern himself with here.