Kopreitan

''“What is the only provocation that could bring about the use of nuclear weapons? Nuclear weapons. What is the priority target for nuclear weapons? Nuclear weapons. What is the only established defense against nuclear weapons? Nuclear weapons. How do we prevent the use of nuclear weapons? By threatening the use of nuclear weapons. And we can't get rid of nuclear weapons, because of nuclear weapons. The intransigence, it seems, is a function of the weapons themselves.”''

- Martin Amis ,  Einstein's Monsters 

A fundamental change to the universe’s laws of physics in comparison to our own makes the atomic chain reactions for a fissile nuclear detonation impossible. Such fission-based chain reactions are the core principle that allows nuclear weapons to function.

In this universe, the Manhattan Project is ultimately a failure, as are all other nuclear weapons projects that follow it, leading to world completely devoid of nuclear weapons.