It runs counter to intuition, but I've come to believe that at least Hiroshima bombing was the lesser of the evils because:

1) As other respondents have said, MANY allied (mostly American) and Japanese lives would have been lost during a sustained invasion of Japan.

2) People forget how fanatical and vicious the Japanese were (I lost a 2nd cousin, a pilot, in Japanese prison camp). They wouldn't surrender and the lives of POWs were also at stake.

3) LeMay wanted to continue to firebomb Tokyo, which would have cost many lives. LeMay may have been "jealous" of the A-bomb strategy

4) Time was of the essence, as Soviets were about to gobble up Manchuria and even parts of Japan proper. Resulting Korean War was bad enough, imagine if they'd taken more territory (it could have tipped Cold War balance, set table for many more lives lost)

5) Also, the macabre "good news" is that no one has used nukes in war since..."mutual assured destruction" a lesson learned and strange kind of peacekeeper?

6) Strange to consider, but in summation the lesser of the evils. This is something to consider today, as what if Russians nuke Ukraine or N Koreans S Korea or Japan? What would be the "measured" response (that would need to end the conflict in one fell swoop?)