People have been making memes about Robert J Oppenheimer and calling the man an idiot for thinking the atomic bomb wouldn’t kill people. What people do not realize is that the atomic bomb was less supposed to be dropped on cities and more supposed to be used as the ultimate shock and awe tactic.
For those who are not familiar with military jargon, shock and awe (or rapid dominance) is a military strategy where you are supposed to show so much force that it scares the enemy on the battlefield and destroys their will to fight while minimizing casualties.
According to historians Harlan K. Ullman and James P. Wade, "the appropriate balance of Shock and Awe must cause ... the threat and fear of action that may shut down all or part of the adversary's society or render his ability to fight useless short of complete physical destruction."
What does this have to do with Oppenheimer? Well, the man said, “Look, we use this thing, and we’ll never have to use it again because it’s so horrible. This will be the end of the war. People will see how terrible these weapons are, and so they won’t use them again.”
However, Oppenheimer also said, “The atomic bomb is sh-t,” describing how bad it would be as a weapon of war due to how much damage it would cause if used directly on settlements & cities, according to historian Evan Thomas.
While Oppenheimer and some scientists were open to using the bomb, the point with shock and awe and why Oppenheimer regrets it is more of the fact that it was dropped directly on cities. The idea of such a big bomb may have been more of having a weapon that could be dropped at a distance from major populations but still produces enough of a yield or explosion to scare people into surrendering simply by seeing the massive power of such a bomb.
Scientists like Oppenheimer and Ernest Lawrence suggested using the bomb in a non-combat demonstration or at least using it miles away from a major settlement like Nagasaki instead of dropping it directly on any highly populated area, allowing the shock to potentially get the Japanese to surrender while minimizing casualties from such action. After all, that is kind of what the bombs were made for--to be the ultimate shock and awe that could be dropped in a remote area, to be seen in major areas of an enemy nation, and convince even the most war-hungry psychopaths to surrender while minimizing casualties that would be caused by a direct bombing.
However, this proposal was rejected with an official statement saying that "we can propose no technical demonstration likely to bring an end to the war; we see no acceptable alternative to direct military use."
Because of this, Hiroshima and Nagasaki were directly bombed instead of being exploded miles away in an attempt to shock enemies into surrender as originally suggested. On top of that, more atomic bombs were made, and the world had to sit through the Cold War since Oppenheimer’s prediction that such a weapon could scare humans into not fighting did not come to pass.
So these reasons are why Oppenheimer became depressed and somewhat regretted the atomic bomb. The weapon that was supposed to be so powerful you could scare people into surrender and end all war without ever blowing up a city was used to blow up two cities, and the end of potential war - nuclear or otherwise - did not come to pass…