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Operation Downfall: The Allies’ Plan for an Invasion of Japan

The Second World War was without a doubt the bloodiest conflict in human history. Most sources agree that across multiple continents over 70 million people were killed, more than half of which were civilians. Among the more infamous events of the war are the bombings of Hiroshima and Nagasaki – the only instances a nuclear weapon has ever been used in combat. The exact figures aren’t known, but most estimates place the total casualties of the atomic bombs above 150,000. It’s quite a common opinion these days to say that these attacks were unjustified, and in some cases, they are even referred to as a war crime. But the bombs did achieve their goal of intimidation, and on August 15, 1945 Japan surrendered to the Allied Forces. But what would have happened if the United States had decided to not drop the atomic bombs, or if Japan hadn’t surrendered? Well, the alternative plan was known as Operation Downfall – a ground invasion of the Japanese mainland. This invasion, thankfully canceled after the Japanese surrender, would have likely become one of the deadliest battles of the entire war.


Originally, the United States wasn’t planning to enter the war at all. The American public was against joining the fight in Europe and opted to remain neutral and support its allies with weapons and ammunition instead. But this all changed in December 1941 when Japan launched a surprise bombing on Pearl Harbor, a navy base in Hawaii.

pearl harbor bomingo
Photograph taken from a Japanese plane during the torpedo attack on ships moored on both sides of Ford Island. View looks about east, with the supply depot, submarine base and fuel tank farm in the right center distance. A torpedo has just hit USS West Virginia on the far side of Ford Island (center). Other battleships moored nearby are (from left): Nevada, Arizona, Tennessee (inboard of West Virginia), Oklahoma (torpedoed and listing) alongside Maryland, and California. On the near side of Ford Island, to the left, are light cruisers Detroit and Raleigh, target and training ship Utah and seaplane tender Tangier. Raleigh and Utah have been torpedoed, and Utah is listing sharply to port. Japanese planes are visible in the right center (over Ford Island) and over the Navy Yard at right. Japanese writing in the lower right states that the photograph was reproduced by authorization of the Navy Ministry. U.S. Naval History and Heritage Command Photograph.

The attack on Pearl Harbor was meant as a preemptive strike. Japan had already invaded China and much of Southeast Asia and was now snatching up pacific islands in an attempt to expand its empire and naval control. Assuming the US would intervene at some point, Japan decided to take the first swing, and with over 2000 American servicemen killed in the attack, and 4 battleships sunk, the United States declared war on the Axis Powers and immediately joined the war.

While the Allies were pushing back against the Axis forces in Europe, much of US attention was on the Pacific Theater. Several missions, such as operation Reckless, Cartwheel, and Persecution, adopted the strategy of “Island Hopping, where the US forces would quickly capture small islands and weaker bases, isolating the more fortified islands from supply routes and leaving them to “wither on the vine”. This strategy was a huge success, and for the next couple years, US forces were steadily recapturing islands under control of the Imperial Japanese Navy, such as Guam and the Philippines. But as Japan centralized its forces on the main islands in 1945, a new approach would be needed to successfully reach Tokyo.

At the time, the development of the atomic bomb was highly classified. It was such a well-kept secret that Vice President Harry Truman wasn’t even aware of the bomb’s existence until he became president in April 1945. And with no idea that a superweapon had been developed, 2 ideas were proposed for attacking Japan.

The first option was proposed by the Navy, who believed the best course of action was to capture strategic airfields in China and Korea. From these bases the US would proceed to bomb Japan into submission while a naval blockade surrounded the country. This idea was ultimately rejected due to worries that it would prolong the war indefinitely, leaving only one option, the Army’s plan. The Army

proposed a direct landing on Japanese beaches, similar to the D Day landings. This was Operation Downfall.

Battle Plan

While the landings at D Day were a great success, the exact same strategies couldn’t be used in Operation Downfall for a number of reasons. Firstly, Japan’s unique geography meant that beach landings were only realistic in a few places, namely Kyushu and the beaches south of Tokyo on the Kanto Plain, meaning the attack was entirely predictable. Another assumption was that the civilian population would also be hostile to the invading Allied forces, something not really seen in European fronts that could seriously impede the attack and drastically increase casualties for both sides.

The US, now aided by Britain and Canada, decided on a two-phase attack. The first phase, scheduled for November of 1945, was nicknamed Operation Olympic and was a full-scale invasion of Kyushu, the southernmost island of mainland Japan. The combined Allied fleets for this landing would have been the largest armada in history, consisting of 400 destroyers, 42 aircraft carriers, and 24 battleships.

For the initial landings, the ground force consisted of 15 US divisions, that’s about 250,000 troops. These teams were to launch from the islands of Okinawa and land in three coastal cities: Ariake, Myazaki, and Kushikino. Between these cities, 35 beaches were selected as targets, and it was estimated that the invasion force would outnumber the Japanese defenders three to one at each landing site. The ground troops would be supported by the Fifth, Seventh, and Thirteenth Air Forces, with strategic bombing by the Eighth and Twentieth, as well as the British Tiger Force.

The goal of Operation Olympic was not to take the entire Island of Kyushu, just the lower third of it. This would give the Allies enough space to build air bases and launch the second phase of the attack: Operation Coronet.

Operation Coronet was the landing on the beaches south of Tokyo, scheduled for March of 1946. The forces for this landing would have been immense – 45 divisions were assigned in two waves,

that’s 675,000 men. For comparison, the initial landings at Normandy used only 12 divisions of men. The goal was to take the beaches, march north, and encircle the capital.

Now, on paper, that all sounds great. Take the southern island, build some bases, and then attack the capital, all while outnumbering the Japanese. But it wasn’t going to be that simple. As mentioned earlier, this attack was predictable, and Japan was prepared for it.

Operation Ketsugo

The Japanese commanders were well aware of the Allied intention to invade the mainland, and Operation Ketsugo was the codename for the defensive preparations on the islands. At first, the Japanese were expecting an Allied invasion in the summer of 1945, but the battle of Okinawa lasted much longer than expected, and it was figured that the Allies wouldn’t be able to muster an invasion force before the mid-summer typhoon season.

By this point, Japan was well-aware that they couldn’t actually win the war. Their navy had taken a serious beating at Okinawa, and they knew they were outnumbered and surrounded by the

Allies. Instead, the plan was to make an invasion so costly that the Allies would settle for some sort of treaty instead of an attack, and Japan would avoid a full surrender of their country.

To achieve the goal of an impenetrable island, the commanders planned to militarize the entireJapanese population. Throughout the summer of 1945, propaganda called “The Glorious Death of One Hundred Million” were spread throughout the country. The overall message was that it was “glorious to die for the holy emperor of Japan, and every Japanese man, woman, and child should die for the Emperor when the Allies arrived”. Similar ideas had already been widespread in Okinawa and were partly the reason the battle was so deadly, as Japanese officials had told the civilians that it was far better to die a patriotic death than to be captured by the “white devils”. Many civilians in Okinawa, unable to fight, committed suicide when faced with capture by Allied troops, and Allied leaders feared a similar response all across mainland Japan.

Allied intelligence also reported in the summer of 1945 that Japanese forces in Kyushu, the location of the first planned beach landings, had grown to almost 600,000, almost triple the allied estimate. In fact, Japan was so confident that the initial battle would take place here, that an estimated 40% of all the ammunition in Japan was stored in Kyushu. Local officials also organized Volunteer Fighting Corps, which consisted of men and women deemed healthy enough to fight. These volunteers numbered almost 28 million throughout Japan, and were equipped with only Molotov cocktails, longbows, and swords, or bamboo spears. They were ordered to conduct guerilla warfare and to serve as a second line of defense to the invasion. One volunteer, a high school age girl, was provided with only a chisel, and was told to aim for the abdomen.

Japan also planned to fight for air superiority over their island. Estimates in June of 1945 showed only 2500 Japanese aircraft ready for combat, but this number quickly grew to 10,000 by August. It was well known to the Allies that nearly half of these aircraft were kamikazes.

Kamikaze pilots were a great concern to the Allied generals, and preparations were made to counter them. One plan involved sending a dummy fleet to the invasion site a week before the actual invasion. This would hopefully lure out hundreds of kamikazes who, expecting transport barges full of troops, would actually be greeted with destroyers lined with anti-aircraft guns. Allied B17 bombers were also converted into flying radar stations, which would direct squadrons of fighters to intercept kamikazes. But kamikazes were only one part of the desperate act: Japan had also prepared Kaiten – human torpedoes, Shinyo – exploding boats, and Fukuryu – human sea mines.

Wild Cards

The expected guerrilla tactics of Japanese civilians was a cause for some commanders to consider chemical warfare throughout Operation Downfall. Chemical weapons, including gas, had been banned in the Geneva Convention following World War 1, but neither Japan nor the United States signed the agreement. Toxic gas was produced and stored in large quantities, but the Allies would have been reluctant to actually use them, fearing retaliation. Instead, bombs were developed with nine different compounds designed to destroy crops, hoping to starve the population into surrender. These chemicals later became the foundation for the infamous Agent Orange in the Vietnam War. But as devastating as chemical warfare would have been, it wasn’t the most dangerous weapon prepared for the invasion.

When news of the atom bomb became public after the bombings of Hiroshima and Nagasaki, Major General John E. Hull, not expecting the Japanese to surrender so quickly, investigated the potential use of strategic atom bombs on Japanese ground forces. At the time, the dangers of

radioactive fallout weren’t as well known, and the plan was to simply wait 48 hours after the blast to occupy the targeted city. Today we know that this would expose invading troops to serious amounts of radiation. When asked about a potential supply of nuclear weapons, Ken Nichols, a district engineer who worked on the Manhattan Project, wrote that if Operation Downfall were to be initiated, the army could be supplied with up to 15 plutonium bombs.

Something completely unknown to the Allied invasion planners was that the Soviet Union also had its own ideas to invade the Japanese mainland. The Soviets intended to invade from the north, which would have the advantage of avoiding most of the Japanese forces that had been transferred to the south. Russia at the time had immense numbers of ground troops, but their navy was lacking, and this would have made an amphibious assault on the beaches difficult. Ultimately the Soviets avoided the main islands, opting to recapture some smaller Japanese islands to the north.

Avoided Tragedy

Fortunately, Operation Downfall was called off after the unconditional surrender of Japan in August 1945. As part of the surrender, Japan handed over the majority of its military equipment and vehicles to the Allies, who realized they had seriously underestimated the Japanese firepower ready to defend the islands. While the number of casualties will always remain an estimate, American officials predicted a total between 1 and 4 million Allied casualties, and between 5 and 10 million Japanese casualties. These estimates were largely based off the battle of Okinawa, where nearly half the civilian population of 300,000 were killed. In fact, in preparation for the invasion, the United States produced 500,000 Purple Heart medals, the award given to soldiers who are killed or injured in combat. These Purple Hearts, unused after the canceled invasion, have since been awarded to American soldiers in the Korean War, Vietnam War, Iraq, and Afghanistan, and in 2003 there were still 120,000 remaining.

Today, Operation Downfall remains a popular topic for alternate history books, such as the 1983 The Burning Mountain, which describes the failure of the atom bomb test and the subsequent invasion of Japan.

The planned attack will always be a central part of the debate regarding the attacks on Hiroshima and Nagasaki, because as devastating as the atom bombs were, and as humiliating as defeat was, Japan was spared one of the most devastating invasions in human history. The decision to surrender to the Allies likely saved millionsof lives.

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