1) To see if it would work as a militaraized weapon, not on some test tower in Alamagordo, NM. The target was picked and the bullseye was to be the HQ parade ground of the 25th? Japanese Division, an elite group brought back from Manchuria recently that were well armed, well disciplined, and well fed (rare for a unit to be at that strength at that time). 70% casualty rate.
2) To prove (and show) Russia that we had the bomb....WEAPONIZED and deployable. Even Stalin said that "That's the only one they had"....Nagasaki sobered the NKVD and Stalin up real quick.
3) To save lives. By using a weapon so terrible that it would force either the IJN/IJA annihilation or surrender, they HAD to use the bomb unless they wanted to lose another 200-600,000 American lives and 2-4 million Japanese soldiers and civilians.
We HAD to drop the bomb, and we should be glad that it did. The deterrence factor alone lasted for decades after it because the world knew that we had the will to use it.
The next nuclear weapon to explode in anger will probably be between India and Pakistan, India and China, Iran and Israel, or a dirty nuke by mooselimbs......all the while Obama is trying to gut our nuclear forces.
The real hard underlying truth is that the two bombs prevented the Japanese people from being virtually annihilated and reduced to the same status of today’s North Korea.
One has only to read the Japanese accounts of the dire circumstances of the home islands population. They were on the verge of starvation. Rations were drastically reduced with no food being imported. They were eating acorns when they could find them. Pulling up ditch weeds for food. Their young men were already dying in untold numbers. The losses of another million or more would have left with with a stunted postwar population with little chance of recovery for generations.
And the Japanese general staff was telling them to gather rocks, to sharpen sticks and farm tools to fend off the coming invaders. This across the land that had been devastated by the huge regular convenional bombing raids that did far greater damage with their incendiary loads. Their industrial base was crumbled into dust. An American naval blockade would have denied them fuel, food, and raw materials. Totally cut off from the rest of the world and winter coming on, a winter that would have killed many, many more. Freezing, starvation and disease was what was in store for them.
The Japanese should mark August 6 as the day that ended the insane war set upon them by their Emperor and his General Staff. Every post war advance and their economic rebirth with the attendant improvement in their health and living standard stems from those two bombs.
My advice for the Japanese is to kneel and bow to the rising sun in the east on August 6 and thank whatever god they worship for the mercy extended to them by the USA.