Seventy-one years after the United States dropped two atomic bombs on Japan, killing about 140,000 people and ending World War II, Barack Obama last week became the first sitting American president to visit Hiroshima, to commemorate what everyone (regardless of whether he or she thinks it was justified) can agree was a devastating tragedy.
And although Obama made it clear in advance that he would not apologize for the bombings, it was something many people felt strongly about one way or the other.
The Question at the Root of It All
Of course, before addressing this, we have to ask whether the U.S. was wrong to use atomic bombs under those circumstances.
This is where military and moral calculations overlap at every turn—in fact, the military considerations are part of the moral ones. Although both Hiroshima and Nagasaki were militarily significant, the atomic bombs targeted civilians, which was contrary to existing moral expectations.
While it is true that over the course of the war, ethical norms had weakened so far that targeting civilians was well within the purview of the momentary moral landscape (e.g., carpet bombing by multiple parties), both the creators and wielders of the bomb recognized that it was technologically transformative—a “destroyer of worlds”—not merely a weapon of greater force. As a result, they should also have recognized that they needed to think about the morality of this weapon differently.
A Common Justification That’s Really Not Much of a Justification
One common justification for the bombings is that it saved lives overall, as it would have been much more costly to wage a land invasion of Japan, which both sides were preparing for. The post-hoc rationale of an estimated 230,000+ American lives saved is misleading however, as military estimates at the time were closer to 63,000. This is before the U.S. learned of additional Japanese preparations against an invasion, so a more accurate projection lies somewhere in between, but most likely still well-southward of the total 200,000+ Japanese casualties from the two bombings.
Furthermore, traditional just war theory requires calculating not just one’s own casualties but rather total damage done to all parties, in order to determine whether the force used is proportional to the ends sought (not to the cost).
On this matter, it is important to note that the unconditional surrender demanded by the U.S. was not the norm then or now, and is unnecessary for military victory; Japan might have surrendered earlier had it been able to negotiate for its terms, and the U.S. was aware of that possibility.
This moral dilemma also cannot be divorced from whether the bombings were militarily necessary to force Japan’s surrender and end the war. On that issue, the inquest yields uncertain results. There is a great deal of disagreement about how to interpret the historical record, and historians are usually split between three main options:

1. Japan would not otherwise have surrendered and this was the only way to end the war and save lives overall (see above).
2. Japan was already discussing surrender and would have been motivated by the USSR’s entry into the Pacific Theatre, which happened between the two bombings. Therefore, the bombings were meant to establish an early advantage vis-à-vis the Soviets in the post-war period.
3. The bombings were primarily to end the war quickly, although may not have been absolutely necessary, with the side benefit of sending a message to the Soviets.

Not Even a Warning
From a moral perspective, even mixed motives are a problem, and what is indefensible is Truman’s decision to not invite Japanese observers to the Trinity test to exhibit the atomic bomb’s power in order to encourage prompt surrender.
The American leadership discussed but rejected the idea of a demonstration because they decided (on Japan’s behalf) that it would not be convincing enough—and also because they worried about the humiliation of a failed test, thus overriding the most basic humanitarian consideration that an enemy should be made aware of the potential use of a calamitous weapon that would target civilians.
Certainly, that would have ruined the bomb’s shock value, but if the goal of its deployment was Japan’s surrender (rather than killing so many civilians that it would accept unconditional surrender, or sending a signal to the Soviets, or any other motive) then some warning was morally obligatory, even if some speculated it unlikely to produce surrender and even if the weapon were justified on “supreme emergency” grounds, which is debatable.
The historical record leaves a lot of questions about the necessity of the atomic bombings, with the second bombing of Nagasaki only three days later—the interim during which the Japanese barely had time to grasp not only the enormous scale of the damage but also what exactly had happened—being much more suspect than the first of Hiroshima.
So, Should Obama Have Apologized?
Given these factors (and that there should be a bias toward not killing in the face of uncertainty), the bombings seem at least questionable enough to have been unjustified. If that is the case, does the American president owe an apology?
This is a tricky question, and survivors themselves are split on whether they want one, although that is independent from whether one should be preferred. On moral grounds, if a wrong has been committed, then an apology is warranted, even if the wrong responded to an original wrong (i.e., Japan starting a war).
Then two apologies are necessary: there is no zero-sum game when it comes to wrongness, blame, responsibility, and apologies in warfare. It is, however, a tough political issue; no apology was given on this occasion, and one is unlikely to come, as the U.S. president must consider several different audiences—his domestic American constituency, the Japanese people, and the rest of the Asian countries—all of whom are deeply divided on this issue.
Politically, it would make sense to tie an American apology to a Japanese apology, whether in conjunction or successively, if one were to be given at all; unfortunately, the geopolitical dynamics in the Asian region make this pre-condition highly unlikely in the near future.