August 6th, 1945. A day that will live in infamy. #Hiroshima75

For context:

"I have to decide Japanese strategy - shall we invade proper or shall we bomb and blockade? That is my hardest decision to date."

- Harry S. Truman

The plan for the invasion of Japan, Operation Downfall, was to begin in November. Gen. Marshall described a two-phase invasion, beginning with Kyushu. He estimated initial casualties to be around 31,000.

Nimitz predicted 49,000 in the first 30 days, 7,000 more than at Normandy. MacArthur's staff predicted 50,000, though Mac himself considered that too high.

General Andrew Goodpaster, one of Marshall's strategists, anticipated the overall invasion would cost "500,000 killed or wounded, and the Japanese ten times that many."

The plan for the invasion called for the bombing to be increased to the absolute maximum, to the point where said one memorandum: "more bombs will be dropped on Japan than were delivered against Germany during the entire European war."

Truman approved the plan.

All that said, using the bomb was not a foregone conclusion. McCloy offered the bomb, suggesting it might provide a "political solution" invalidating the whole invasion.

"I said I would tell the Japanese we have the bomb, and what kind of weapon it is. And then I would tell them the surrender terms." He thought the Japanese should also be told they could keep their emperor. What if they refused, he was asked.

"Our moral position will be stronger if we give them warning."

Truman indicated he would think bout it.

The ultimate decision was made in late July, at the "Nightmare House" in Potsdam where Truman met with Stalin.

On July 16th, Truman relieved a coded telegram about the Alamogordo test, the first nuclear explosion in history: "Diagnosis not yet complete but results seem satisfactory and already exceed expectations."