VArious people have tried to argue that Truman was wrong tp use nukes
to end WW2, they argue that either Japamn was ready to surrender or
that a demo of the nukes would have been effective.

We can dismiss the idea of a nuke Demo because where would it have been
demo'ed? No location except on a city would have any meaning. A visit
to Alamagordo during the Trinity test would have failed to impress any
more than watching the firestorms over other Japanese cities. The
Japanese people would never have witnessed such a demo, just the
military Junta who were ready to fight to the end. Besides, there were
not enough weapons for such a test. After Nagasaki, it was expected to
be two weeks before another was ready.

Were the Japanese ready to surrender? Some people point to an attempt
by the Japanese ambassador to Russia to discuss terms. If the Japanese
wanted to surrender, they could have contacted any US embassy instead
of the Russians who wwere known to have their own motives. Clearly
this was not a real attempt.
Most people do not know that the actual terms of surrender were nearly
identical to terms that had been communicated months before from the
Potsdam Declaration. The Japanese simplky needed a way to save face
and the use of the A bombs gave them the excuse to surrender under the
same terms previously proposed.

The Japanese junta was so unwilling to surrender, that after Hiroshima,
they had a vote and were tied about surrender. They had to take tyhe
unprecedented step of getting the emporers vote who did not vote to
surrender until after Nagasaki. Even then the Hawks on the junta tried
to stage a coup against the doves so they could continue. When that
failed, they tried to steal the recording of the surrender by the
Emperor. When many japanese heard the emperors voice, many at first
thought they would hear a declaration of VICTORY, not surrender, that
is how well controlled their thoughts were by the junta.

What would have happened without the bombs? The casualties would have
been massive. Casualty estimates were based on those of Okinawa but
failed to take into account the actual readiness of the Japanese to
defend their islands. The US simply did not know the real numbers of
kamikazes that would be ysed against them. The US did not know of the
thousands of suicide frogmen trained to destroy landing craft. Tha
japanese strategy was to attack troop transports knowing tht the US was
tired of war and they thought that if the casualties were high enough,
they could get better terms. Anybody who has seen the pictures of
midget suicide subs stretching far into the distance will realize the
level of difficulty the US faced.
Would the entry of the Soviets have meant anything? It would mean
something but the soviets were not willing to allow the US to use their
territory. B29 crews who reached soviet territory were held prisoner.
In spite of having many men, the Soviets were unable to directly attack
the islands but instead were tied down in Manchuria. The home islands
were safe from any Soviet attack.
Japanese who are familiar with the decisions being made in the last few
months seem to agree that the use of the bombs saved lives.
Other reasons to use the bomb. When Berlin fell, a U-boat was
intercepted in mid-Atlantic carrying a load of unenriched uranium to
Japan. The Japanese officers aboard had killed themselves when the
U-boat surrendered. There was a fear that the Japanese were attempting
to build a dirty bomb to use against the US to be delivered by aircraft
from a submarine. Achieving surrender quickly would alleviate this
very real concern. The Japanese rerally could have done this as they
were experimenting with enrichment using the same method Saddam Hussein
used before Gulf War 1. This would not have been a nuclear explosion
but simply a dispersal of radioactive material.
The use of the bombs was a very public demo of the power of the bomb.
It was really no different than what was planned for other japanese
cities, in fact, the firebombing of Tokyo killed more people. One by
one japanese cities were being destroyed by conventional firebombing so
the use of the a bomb was really no different except that it showed
what ONE plane could do. This gave the Japanese a very powerful way to
save face by surrendering under the previously rejected terms.

I am very sorry I posted this here. I intended to post it to
Sci.military.naval. Please ignore it as it is not about boats.