Which event convinced president truman to drop the atomic bomb?

There are a couple of events that led to the President Truman's verdict to drop the atomic bomb:

1) Japanese leaders failed to heed the warnings and surrender.

2) Japanese belief that they were the 'chosen' people, and would fight to the death to allow the Japanese 'Empire' to continue to survive.

3) Operation Downfall, Olympic, & Overlord proved to be costly and had a high chance of having large amount of casualties.

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what are the options?