They did drop generic leaflets warning of generic (conventional) bombing on many cities. They did not specifically warn Hiroshima of any specific attack, or make any specific warning about the atomic bomb.
Given that the atomic bomb was secret, "prompt and utter destruction" is only a warning in retrospect. There is no way that you can read the Potsdam declaration and think, "oh, they are going to start dropping atomic bombs on cities soon," if you don't know what an atomic bomb is or that they exist.
I wonder how important "warning" really is though if it is this vague. Imagine country A is at war with country B. Country A issues an announcement: "if country B does not do what we want, we will set off a bomb in one of country B's major cities." If they follow through with it, does this sort of warning absolve them of responsibility? Does the presence or absence of this kind of vague warning have any effect either way on the ethical calculus regarding the targeting of civilians?
It seems to me that if it is ethical then it is ethical without the warning, and if it is not ethical then it is not ethical with the warning. Which makes the warning question a red herring.
The firebombing of Tokyo (one of many examples - many Japanese cities suffered mass destruction in conventional bombings) was just as, if not more, horrific than Hiroshima or Nagasaki. Why would the residents care whether they were being firebombed or nuclear bombed?
Given that the atomic bomb was secret, "prompt and utter destruction" is only a warning in retrospect. There is no way that you can read the Potsdam declaration and think, "oh, they are going to start dropping atomic bombs on cities soon," if you don't know what an atomic bomb is or that they exist.
I wonder how important "warning" really is though if it is this vague. Imagine country A is at war with country B. Country A issues an announcement: "if country B does not do what we want, we will set off a bomb in one of country B's major cities." If they follow through with it, does this sort of warning absolve them of responsibility? Does the presence or absence of this kind of vague warning have any effect either way on the ethical calculus regarding the targeting of civilians?
It seems to me that if it is ethical then it is ethical without the warning, and if it is not ethical then it is not ethical with the warning. Which makes the warning question a red herring.