I doubt very much that [American] casualties would have reached anywhere near [the one million] figure. The cream of the Japanese army had largely died in previous battles, or was stranded on isolated islands. By summer 1945, they were reduced to arming civilians with pointed bamboo sticks. Shortages of everything from food to lubricants were getting desperate. There was virtually no fuel, the railway system was collapsing, as were communications. The Japanese had very little ability to defend against armour or air attack.
Furthermore, had the war continued into the fall of 1945, the Soviets would have invaded from the north, so the Japanese would have an extra front to defend.
That's not to say it would have been a cake-walk. Undoubtedly thousands or tens of thousands of Americans would have died [certainly tens of thousands; even revisionist historians attempting to discredit the bombing speak of "only" 25,000 dead—DF], and many more been wounded. But even in the spring of 1945 an increasing percentage of Japanese troops were starting to surrender. The Japanese are not stupid people, and were coming to accept that the promises of victory (or at least, fighting the Americans to a stalemate) had been lies.
General Marshall knew prior to the February 1945 Yalta Conference that Russia would break its nonaggression pact with Japan and attack Manchuria about 90 days after the surrender of Germany (VE Day) The Magic Summaries documented the shift of Soviet troops by rail from Europe to the Far East for this purpose. We also knew from the Magic decrypts that the Japanese home islands were to be defended from invasion and occupation by 2.3 million troops, another four million Army and Navy employees and a newly created armed militia numbering 25 million. These defenders were sworn to fight to the death, which so many Japanese troops had done in battles throughout the Pacific.
To effectively invade and occupy Japan, American strategists foresaw two invasions, scheduled for November 1945 and March 1946. The first invasion, on the island of Kyushu would employ some 770,000 American troops. The follow-up invasion on the plains of Tokyo, leading to the forced occupation of Japan, called for two million American troops.
But according to documents that have been uncovered, a conference to discuss pre-invasion casualties was held at the White House on June 18, 1945, between President Truman and the Joint Chiefs of Staff. From the Pacific, Gen. Douglas MacArthur submitted rather optimistic casualty estimates. This caused Adm. William D. Leahy, Truman's military advisor, to take charge of the session. Based on the experience at Iwo Jima and Okinawa, Leahy predicted that in an invasion of Japan, 30% to 35% of US soldiers would be killed or wounded during the first 30 days. Truman obviously understood what Leahy said. The president remarked that the invasion would create another Okinawa from one end of Japan to the other. The Joint Chiefs of Staff agreed.
Suddenly, and only after being advised about the buildup of Japanese forces and fortifications by Magic intelligence, MacArthur's medical staff revised its pre-invasion needs for hospital beds upwards by 300%. MacArthur's chief surgeon, Brig. Gen. Guy Denit, estimated that a 120-day campaign to invade and occupy only the island of Kyushu would result in 395,000 casualties.
On July 4, 1945, the British agreed to the use of the atomic bomb against Japan. On July 16, during the Potsdam Conference, the first A-bomb was successfully tested. A way had been found to end the war quickly and decisively. This was the situation on July 26 when the US, Britain and China issued the Potsdam Declaration to Japan to surrender unconditionally, "The alternative," said the declaration, "is complete and utter destruction."
This is what the Americans President Truman, Secretary of War Stimson and Gen. Marshall knew the day before the first atom bomb fell on Japan. Confronted by an enemy leadership that was self-deluded, neither prepared to surrender nor to negotiate seriously, the Americans decided that the only way to end the war quickly would be to use overwhelming force: nuclear weapons.
Two bombs were dropped. The Russians invaded Manchuria. On August 14, Emperor Hirohito overruled his militarist advisors and accepted the Potsdam declaration. Japan surrendered.
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