When we discuss the concept of "imminent threat", we must also be aware of the wartime concepts of momentum, and the fact that many Asian countries conceive war in terms of centuries, unlike America which conceives of war in terms of months or years.
Given the fact that we were engaged in a certain momentum with resources allocated at great expense, combined with the known propensity for Japan to conceive of war as lasting centuries, I think it was absolutely necessary to drop the bombs.
Of course there existed a punitive aspect to the decision, but those who understand war understand that Japan could have easily cost us another half million lives, finally surrendered, then over the course of the next century rebuilt and re-armed. There would be a score to settle, and the fact that 100 years had passed would have been insignificant to them. That passage of time would have only served to soften us and make us complacent and therefore vulnerable.
Warfare is about crushing your enemy so viciously that they don't even have a shred of a chance of recovery. That is called guaranteeing our survival.
We failed to do this with Germany. Now they are quietly manufacturing vast amounts of military hardware and selling it to other countries. They have a completely deployed infrastructure for rapidly rearming to an alarming degree. Great Britain, who has been watching this idiot Obama turn America' back on their staunchest ally with both actions and insults, has been pressuring Germany to rearm and extend its reach via air and sea. The U.S. has also been pressuring Germany to do the same.
In Great Britain's and America's ignorance, they think that WWII is actually over. It's not... we're simply enjoying a temporary cease-fire. In their ignorance, they think that 60 years of cease-fire means the smoldering embers have died out, when in fact the fire is poised to burn again.
Germany will rise again (militarily), because we did not fully crush them back in the 40s. Japan will not rise again because we completely brought them to their knees to the point of near-annihilation and the infusion of our culture into theirs.
When we consider "imminent threat", we need to expand our awareness beyond a few years and consider the next 3 centuries. That's what Asia and some parts of Europe do.
+1 for the A-bomb. The times and the context demanded it.