We should offer peace because peace doesn't involve killing innocent civilians. It is not necessary to obliterate people in order get their surrender and in many cases unconditional surrender is not necessary in order for peace to happen.

"...from time to time there will have to be exceptions not to the surrender principle but to the application of it in specific cases..." (Letter from President Roosevelt to Secretary of State Cordell Hull, April 5, 1944)

The Japanese could not have fought for much longer, and most knew that they had lost. Plus, the lack of proportionality between Pearl Harbor and Hiroshima caused the United States to lose their moral authority.

"...a considerable portion of the Japanese population now consider absolute military defeat to be probable." (Report from Combined British-American Intelligence Committee to the Combined Chiefs of Staff, July 8, 1945)

"We have great moral superiority through being the victim of her first sneak attack..." (Memorandum from Secretary of War to President Truman, July 2, 1945)

The number of American deaths at Pearl Harbor was 2,500. (History.com)

At Hiroshima alone, there were 90,000 to 120,000 deaths from the initial hit and the aftermath of the atomic bomb. ("Children of the Atomic Bomb" UCLA.com)

Demonstrating the power of the bomb would have not only been a waste, considering the huge financial investment into the bomb, but it also would have spurred other countries to start building their own nuclear programs.

"Average cost per atomic device/bomb: $5 billion" (Debate on the pros and cons of dropping the atomic bomb on Japan)

By 1949, the Soviet Union had exploded and tested its first atomic bomb. (The Cold War Museum)

Because Japan would have surrendered anyway, and because the use of nuclear weapons is immoral in any situation, the atomic bombing of Japan was not necessary.

"Japan has no allies. Her navy is nearly destroyed..." (Memorandum from Secretary of War to President Truman, July 2,1945)

"Radiation-induced cancers will affect many, often over twenty years later."

("The Effects of Nuclear Weapons" Campaign For Nuclear Disarmament)

"Nuclear weapons cause severe damage to the climate and environment on a scale incomparable to any other weapon"

("The Effects of Nuclear Weapons" Campaign For Nuclear Disarmament)