It's not an easy question to answer and historians have argued about it for awhile.

To put it simply, it is fairly easy to explain the bombing of Hiroshima in terms of strategy and planning. It is well-documented. There are a lot of different motivations, including thoughts about the postwar situation (which includes the Soviets).

The second bomb (intended for Kokura but dropped on Nagasaki instead) is more troublesome. In hindsight, the official narrative was always about how, well, one bomb didn't do it, maybe a second was needed.

But there is no real evidence from the time that anybody thought that way. And it misunderstands the role of "strategy" in the actual dropping of the bombs — the question of a second bomb was never deliberated at the high levels.

The way it worked is something like this. The idea was to have several bombs to use, eventually, but most strategic discussion focused on the first bomb. The official bombing authorization of late July 1945, however, said that:

Additional bombs will be delivered on the above targets as soon as made ready by the project staff. Further instructions will be issued concerning targets other than those listed above.

So it was an extremely open-ended order. The initial bombing date was "after about 3 August 1945," and the first date that gave good weather for the bombing was August 6th. That became the Hiroshima bombing. The next one was scheduled for sometime around August 11th, but the weather forecast looked poor, so it was dropped on August 9th instead. Which is to say, the close proximity of the two bombs — within 3 days of each other, as opposed to 5 days of each other — was not due to high strategy, but to weather forecasts, and was decided by the people on the ground on the island of Tinian, not politicians in Washington.

This is of relevance to the question because there are a lot of people who think the bombing of Hiroshima is justified, but when you get into the question of Nagasaki it gets more murky. Three days is not adequate time for a government to make a major decision; it is not really even adequate time for a government to assess what has happened (whether it was really an atomic bomb, what the damages were like, etc.).

I think it is telling that immediately after Nagasaki, Truman issues an order that said, in essence, no more atomic bombing until I authorize it explicitly. (And there were additional bombs being readied.) I see this as a re-assertion of control from someone who realizes that major geopolitical decisions are being made by local airmen.

There are different interpretations for why two bombs were slated to be used at all. Personally I lean towards the one that it wasn't actually part of some grand design. They had two bombs ready to go and they had a blanket order to bomb as they saw fit and they used them as such. I don't think it was due to high strategy whatsoever — the people who made the decisions on the ground were not concerned with Russia or anything else; they didn't even particularly regard the bomb as being something unusual, just another weapon.

Well it certainly wasn't to scare the Russians - Truman informed Stalin of the 'powerful new weapon' at the Potsdam conference, and I distinctly recall reading that Stalin gave Truman the impression that he was already aware of it, but I can't recall the source for that. In any case the immediate concern was to get Japan to surrender before they gave much - or any - ground to the Soviets. Any posturing versus the Soviets was already done by simply making Stalin aware that the Americans had built a working bomb.

In Japan's Longest Day, it is made very clear that:

• The Japanese Supreme War Council was quite aware of the coming Cold War, and that their earlier or later surrender would put tilt the balance in favor of or against the Americans accordingly.

• That they understood that this was a new weapon now and forevermore a part of the world stage. This is reflected both in that they had an (extremely crude) idea of how the bomb worked the day it was dropped on them, and the Emperor using it to save face in surrender.

• The complete whiff of a response Nagasaki generates (it 'changed no opinions') suggests that the idea that the Americans 'might only have one' didn't cross the minds of the leadership of any major world power. It's a complete non-thought. It feels like a later invention to try to justify the second bomb.

This is a very difficult question, and one that's open to speculation, interpretation and opinion. However, there is evidence that even though Emperor Hirohito was ready to capitulate, there was dissension amongst his war cabinet.

The way the surrender went was that Hirohito created a recording that was then to be broadcast nation wide in Japan, and between the time it was made and the time it was broadcast, there was a plot amongst his cabinet members to steal the recording and not surrender. It was called the Kyujou incident. (1)

In fact, the emperor was not in control of the war efforts, and many times feared for his own life during the war, fearing a coup d'état from his war cabinet. (2)(3)

Of course none of this means that the second bomb was justified, or that the US didn't also want to send a message to the Soviet Union, knowing they were also very close to developing a nuclear weapon. This is just to illustrate that surely intelligence reaching the US president and cabinet knew that it was not a black and white situation.

(1) Source: https://en.wikipedia.org/wiki/Ky%C5%ABj%C5%8D_incident

(2) Source: book "ano sensou wa nad datta no ka (あの戦争は何だったのか。)" (book is in Japanese) https://www.shinchosha.co.jp/wadainohon/610125/

(3) Source: http://www.nytimes.com/1990/11/15/world/in-a-memoir-hirohito-talks-of-pearl-harbor.html

I've always thought they could blow up the 2nd one any where (say a forest). And go to Japan "we can keep on nuking you, but we want to stop the killing and end the war".

If memory serves me correctly, Japan thought the first one was a natural accident. I also believe the nukes saved Japanese lives (more Japanese would have died from a land invasion, besides the obvious American lives).