Would Operation Downfall have been the largest and costliest battle in history if it was actually executed?

Answer 1

Operation Downfall, the planned Allied invasion of Japan at the end of WWII, would be classified in military nomenclature as a "campaign" involving several pitched battles between the invading Allied and defending Japanese forces. In terms of a single battle, this is a difficult question to answer, as the operation itself never took place. However, by analyzing both US and Japanese plans for attacking/defending the home islands, we can get a fairly good idea of how the campaign would have played out.

The first phase of "Downfall," Operation Olympic, targeted the southernmost Japanese Home Island of Kyushu. According to D.M. Giangreco's 2009 book "Hell to Pay: Operation Downfall and the invasion of Japan, 1946-1947," the forces assigned to the US Sixth Army, which was given the task of securing approximately one-third of that island, would have numbered roughly 766,700 in 14 divisions (with apparent provision for reinforcement by six additional divisions), while the defending Japanese 16th Area Army (Army Group/Front Equivalent) numbered about 917,000 in 15 divisions (plus an antiaircraft division) and 10 brigades, plus the Iki and Tsushima fortress units. The Japanese, if need arose, also had provision for reinforcements of up to four additional divisions from Honshu.

However, as mentioned above, 'Olympic' as a whole represents not a battle, but a campaign.

The most plausible prospect of seeing a 'record-breaking' single battle rests with the second phase of "Downfall:" Operation Coronet, the invasion of the Tokyo area at the Kanto Plain.

In order to achieve final victory over Japan, the Allies realized they needed to control Tokyo. Since the Japanese realized not only this blatant Allied objective, but also the rough US plan of invasion, they organized their forces to ensure that the 'Yankee Barbarians' would be crushed by one fell swoop on Honshu. According to Giangreco, the initial Allied invasion of Honshu during 'Coronet' would consist of the US First and Eighth Armies, organized into 28 divisions and 1,069,000 ground and air personnel, with reinforcements arriving in the form of 12 divisions of the US Tenth Army and the British "Commonwealth Corps," comprising an unknown, but presumably large, number of personnel. To defend Tokyo, the Japanese 12th Area Army could muster 22 divisions and 14 brigades of unknown numerical strength (Giangreco cites total Japanese regular troop strength in the Home islands as roughly 2,900,000. With ~900,000 already earmarked for the defense of Kyushu, it is reasonable to estimate that at least half of the remainder would be somewhat involved in the climactic struggle for the capital), plus plans for reinforcement by as many as 5 additional divisions from areas outside the Area Army's jurisdiction. Since this military operation would focus almost exclusively on the capture/defense of Tokyo, and was referred to by both sides as a coming 'decisive battle,' it could be fairly be referred to as such.

Thus, the final balance sheet in the hypothetical "Battle of Tokyo" would have looked something like this:

12th Area Army (Japan): 27 divisions and 14 brigades

vs

1st, 10th, and 8th Armies (USA/British Commonwealth): 40 divisions plus unknown smaller formations.

It is also important to note that the Japanese also had access to a large pool of civilian militia, called the Patriotic Citizens' Fighting Corps. Giangreco puts the strength of this organization at roughly 28 million(!), compared to the total Japanese population of ~73,000,000. This is in addition to the 1,300,000 civilians already working for the IJN and 2,250,000 working for the IJA. Such huge numbers of militia and paramilitary personnel would ensure that any large-scale confrontation would be unimaginably bloody, on top of the massive civilian casualties that would inevitably result from the fighting in the Tokyo Metropolitan Area.

Thus, it can be safely assumed that the cancelled Invasion of Japan might have produced a land battle that could be called the 'largest' and 'bloodiest' of all time, given the data we have on the opposing forces in terms of their raw numerical strength.