The atomic bomb is a notable example, and consider Eisenhower and many other military leaders said "there was no reason to use that awful thing." I realize my high school education, and most othet's, led me to believe this wasn't the case.
I'm not trying to say it's wrong, but it deserves a lot of skepticism.
It is difficult to question the possible outcomes if different choices were made but you can be sure that the last 80 years could have been much different and much worse. The bomb did make certain kinds of war impossible, and it is not clear if that would be true if they had not been used when they were.
The bomb were used on an enemy that was sooo defeated that there was a coup attempt by the military between the two bombs to prevent the emperor from capitulating.