>destroy world main cities
>blame it on Dr. Manhattan
>unite the superpowers in face of a common enemy
>peace ?

Too many wrong things with this plan

1. the common enemy - that's not a good basis for a lasting alliance. Example: USSR and US were allies as long as they fought against Germany, but become enemies afterwards. And since Manhattan is off to another galaxy, it won't be long before the Cold War starts over again, this time worse because an important deterrence element is missing

2. Dr. Manhattan had been working a long time for the US. I find it hard to believe that the moment they learned the american pet hero was responsible for the destruction of Moscow, the Soviets wouldn't panick and press the launch missiles button