1945's American-only atomic bomb was truly plenticidal (plentitude-reducing), but not because a full-out nuclear war would kill off thousands of the world's species in a few months.
It would have been just as plenticidal if it had never ever been used, but only threatened to be used.
Because its real target was not the Axis or even erstwhile allies like Russia - but rather Washington's favoured peace'n'wartime Allies !
Yes its real target were all the other bothersome friendly nations that Washington had to waste some much time mollifying all throughout the war.
But holding the world's only super-weapon puts an end to all that back and forth diplomacy and compromising and debating.
Conducting external affairs gets much simpler when you hold all the cards, make all the rules.
The Manhattan atomic Project was pursued at all speed and all costs, not because it would end the Axis coalition - but rather because it would end the Allied coalition.
Bet you don't find that fact in any of your professors' favourite books ....