too good to leave in comments, reader and friend “UChuck” elaborates on the atomic bombs for Japan:
As Germany was ground between Allied armies, command in the Pacific was reorganized for the invasion of the Japanese Home Islands. Douglas MacArthur was put in command of all ground forces, including the Marines, while Chester Nimitz took control of all Allied naval forces. Carl Spaatz took over 20th Air Force. Japan would be taken in two operations. OLYMPIC aimed to take the southernmost of the big islands, Kyushu, with a quarter-million men staging from Okinawa. This operation was scheduled to commence in early November 1945. Southern Kyushu in turn would serve as the staging ground for CORONET, an invasion of Honshu aimed ultimately at Tokyo with 300,000 men. It was projected to start about 1 March 1946. Additional men would be pumped into Japan until Japanese resistance ended, possibly as late as 1947. All of these plans were rendered moot when an ultra-secret research project bore fruit in the summer of 1945.
The Manhattan Project originated when a group of physicists approached the government in late 1939 regarding the military potential of the newly-discovered principles of atomic fission. A $6,000 research appropriation in February 1940 grew into a massive research project after American entry into the war. Research labs, pilot production facilities, and finally large-scale processing and manufacturing plants were constructed as the project evolved. Two different bomb designs based off two different fissionable materials evolved simultaneously. Uranium- 235 was processed at a vast new plant built in Oak Ridge, Tennessee, while plutonium-239 was manufactured in a newly-built breeder reactor at the Hanford Engineer Works on the Columbia River in Washington state. All of these facilities were beyond the cutting edge of science and technology. By Summer 1945 Hanford had produced enough plutonium to construct a test weapon to see if all the theoretical work actually worked, and Oak Ridge had processed enough U-235 for a single prototype. The Manhattan Project at this point had consumed over $2 billion. “Gadget,” a plutonium test-bomb, was placed on a steel tower bristling with scientific equipment in a remote corner of Alamogordo Air Base in New Mexico.
Scientists and a few government officials watched from concrete bunkers 10,000 yards away from Ground Zero as the firing switch was closed at 5:30 a.m. on 16 July 1945. A sun-bright flash followed by a wave of heat preceded the roar of the shockwave thundering past the bunker. The tower was vaporized and the sand of the desert was fused into glass to a radius of 800 yards by the fireball as the bomb exploded with the force equivalent to 15,000 to 20,000 tons of trinitrotoluene (TNT) conventional explosive. With the theory confirmed, final assembly on two working aerial bombs was possible. “Little Boy” used a gun design to fire one subcritical mass of uranium-235 into another, while “Fat Man” used a spherical shaped charge to crush a core of plutonium-239 into critical mass to initiate fission. These weapons used up all the available fissionable materials.
American President Franklin Roosevelt died of natural causes on 12 April 1945. The Manhattan Project was so secret that his Vice President, Harry S Truman, had no idea it even existed until he was sworn in as president. Now Truman faced the decision of how to employ the new weapon. Some advised him to detonate a bomb as a demonstration, inviting Japanese observers to witness its destructive power. But if the bomb failed to detonate, or fizzled in a partial explosion, it would be an international embarrassment and stiffen Japanese resolve to fight on. It would also halve the American atomic inventory, and it had taken months of intense effort to produce the fissionable elements in these bombs. Truman weighed all of the factors carefully before deciding the bombs would be dropped in anger. If they prompted a Japanese surrender without an invasion of the Home Islands, American and Japanese lives would be spared. It held the political advantage of ending the war quickly, sure to please American voters weary of rationing, increasing national debt…and sending sons off to fight. It satisfied a national desire for revenge and retaliation against the country responsible for the surprise attack on Pearl Harbor that dragged the United States into this war. It provided a diplomatic advantage in negotiations with a Soviet Union that grew less cooperative once the common Nazi foe collapsed.
The Japanese city of Hiroshima in southern Honshu remained untouched by Curtis LeMay’s 20th Air Force, despite being headquarters of the army defending Kyushu and home to several war industries. It was a compact city of about 290,000 people built on the delta of the Ota River. These factors combined to make it a prime target for the first atomic air raid. With a garrison of 43,000 troops and a number of military factories, it was a viable military target. It had suffered no previous bomb damage, which served a two-fold purpose: it allowed an accurate assessment of the weapon’s destructive capacity, and prevented the Japanese government dismissing the power of the weapon through attributing much of the damage to earlier conventional bombing. American strategists hoped its utter and instantaneous destruction by a single bomb, delivered by a single aircraft, would horrify the Japanese government and prompt a rapid surrender.
Air raid sirens howled across Hiroshima early on the morning of Monday 6 August, but this was not unusual; 20th Air Force carried out daily weather reconnaissance flights over Japan. They sounded again shortly after 8:00 a.m. as a flight of three B-29s of the 509th Composite Group approached the city at 31,000 feet. The Enola Gay, loaded with the 10,000-lb “Little Boy” uranium bomb was flanked at 5 miles distance on either side by The Great Artiste and Necessary Evil, acting as observers to take photographs and record scientific data. The 509th had been practicing precision bombing with heavy ordnance, dropping “pumpkin bombs” – conventional high explosive bombs that mimicked the size, weight, an aerodynamics of the atomic weapons – on Japanese targets in July.
Enola Gay dropped the bomb at approximately 8:14 a.m. Saipan time. “Little Boy” detonated at 1,900 feet over city center with an estimated yield of 15 kilotons. The blast killed about 70,000 people outright from immediate effects, and another 70,000 died of injuries or radiation poisoning by the end of the year. It burned out over four square miles of the city completely. This was not an uncommon casualty figure for a major air raid in Japan – the conventional incendiary raid on Tokyo in March killed over 80,000 and injured another 40,000 – but this had been dealt by a single plane with a single bomb.
Initially, all the Tokyo government knew was that all radio and telegraph communication with Hiroshima abruptly ceased at 8:15. They were perplexed as reports of a massive explosion and widespread destruction began to filter in, as they knew there had been no large scale air raid in southern Japan that morning. A staff officer dispatched to perform an aerial reconnaissance of the city reported a pall of smoke over Hiroshima visible from almost 100 miles away. When the United States officially announced the atomic bombing of the city around midnight Tokyo time, some officers dismissed it as pure propaganda, that such a weapon was impossible. Once better reports of the events of 6 August and the extent of the damage reached Tokyo on 7 August, these officers claimed the enemy could not possibly have more of these weapons. The Japanese government remained officially silent on the development while bitter disagreements raged behind closed doors. There was no Japanese request to negotiate surrender.
The blast did spur the Soviet government into action, however. Stalin had dragged his heels on the promise to make war on Japan once Germany fell. Now the Soviets rushed to declare war on 8 August, to secure a position in post-war settlement in East Asia before they lost the opportunity. Red Army units in Siberia rolled across the border of Manchukuo on the morning of 9 August. (According to Soviet histories of the “Great Patriotic War,” as the USSR called World War II, this Russian declaration of war was the real reason Japan surrendered.)
The United States dropped its sole remaining weapon, the “Fat Man” plutonium-based bomb that same day. The Bockscar found the primary target, Kokura, obscured by clouds, so the aircraft preceded to its secondary target, Nagasaki. The bomb detonated at 1,650 feet at 11:02 a.m. with a yield of 21 kilotons. The hilly geography of Nagasaki put part of the city in the “blast shadow” of a mountain, reducing damage and loss of life. The result was nonetheless devastating, with some 40,000 killed outright, a similar number injured, and two square miles of the city obliterated. The second blast also destroyed the argument that the Americans had but one such weapon.
The Japanese government communicated its willingness to entertain surrender term to the Allies through the neutral Swiss embassy on 10 August. They agreed to a “conditional unconditional” surrender: 1) the person and dignity of the Emperor Hirohito, the Son of Heaven, would remain inviolate, and 2) the Empire would retain the Home Islands intact. Despite an attempted assassination of Prime Minister Suzuki Kantaro by a clique of diehard military officers, the arrangements for a cease fire under these provisions were finalized on 14 August 1945. A somber emperor Hirohito addressed his subjects by radio, announcing
“We have ordered our government to communicate to the governments of the United States, Great Britain, China, and the Soviet Union that our empire accepts the provisions of their Joint Declaration. …it is according to the dictate of time and fate that we have resolved to pave the way for a grand peace for all the generations to come by enduring the unendurable and suffering what is insufferable.”
Their formal surrender was accepted by Douglas MacArthur on the deck of the USS Missouri, anchored with much of the U.S. Pacific Fleet in Tokyo Bay, on 2 September. This was followed with a formal surrender to China in Nanking, on 9 September 1945. The most destructive, most expensive, and deadliest war in human history was over.
(And I extend my thanks to UCHuck for taking the time to post!)