Curious to see how the deep doctrinal changes needed for the IJA to carry this out happen, especially with no war in China.
Ask and ye shall receive...
[lightly edited from a couple of online source - not THAT much is changed ITTL]
For the IJA the Infantry Branch was the heart and soul of the force. Only the best officers were commissioned into the infantry. The bulk of the force structure remained predominantly infantry even as the army modernized in the 1920s and 1930s. In 1917, 68 percent of the force was infantry. Twenty years later, a square Japanese infantry division still counted about 48 percent infantry rifle strength (5600 out of 11,858), and its wartime rifle strength stood at 37 percent despite extensive augmentation of what we today term combat service and combat service support troops.
Anticipating a war against either China or the USSR, which was bound to take place over vast distances and stretch logistics to breaking point, the army created triangular divisions. These units were 43 percent infantry (3780 of 8871). Moreover, in the post–World War I military reductions between 1922 and 1925, the Japanese army opted for a large-scale reduction of artillery firepower, eliminating 115 artillery batteries in 1922 and twenty-four more in 1915. As one former Imperial Army staff officer and historian remarked, "Those thinking that the way to strengthen the division's fighting strength was by eliminating the field artillery brigade and independent mountain artillery regiment probably were influenced by the 1908 Infantry Manual." The doctrine espoused in that particular manual, weighted heavily on the intangible factors of infantry in battle, marked the beginning of a consistent theme in Japanese military thought that infantry, properly led and motivated, can overcome the material advantages of the foe. The Japanese army then was diverging from machine and firepower solutions to tactical problems. Men, not machines and firepower, win wars.
The introduction of the triangular division increased the overall number of infantry divisions available and allowed for a more aggressive us of available forces. Japanese planners envisioned the triangular division as having one sub-unit for a holding attack, one sub-unit for a flanking envelopment attack, and the last sub-unit as an exploitation force. Whereas the square division had focused on firepower the new triangular division fit the strategy of maneuvering quickly and destroying the enemy.
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Primarily, the Imperial Army doctrine foresaw a war against the USSR on the plains of Manchuria. The previously mentioned 1938 manual emphasized morale factors and tenacity of the will and stressed training designed to develop the command ability to recognize opportunities for attack and make quick decisions. The wide-open plains of Manchuria offered Japanese officers opportunity to envelop open Soviet flanks, encircle formations, and destroy numerically and technologically superior forces. Japanese infantry always probed for an open flank to exploit. Since 1908 the traditional Imperial Army emphasis on highly motivated infantry conducting night attacks pressed home with the bayonet had never varied and had served as the basic tactical doctrine of the Japanese Army to counter the superior firepower of the Red Army. Training then emphasized the unique role of the infantryman. He was expected to be tough and resolute and to fight without elaborate combined arms support.
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Tactics and tactical instruction—battle drills and so forth—are products of training designed to implement instinctive reaction. Command of large units cannot be accomplished by reflex. It requires years of education and experience to produce commanders capable of innovative thought and creativity on the battlefield. Yet in the IJA "the heart of the War College curriculum was tactics," although its mission was to prepare officers for high-level command. A typical class for the two-year course between 1915 and 1938, whose graduates became the World War II leaders, had about fifty officers. They were normally captains and majors who were eligible for selection after eight years' service, including at least two with a unit. Subject matter, for the most part, relied on rote memorization of facts, which similar to training, can produce narrow, if effective, specialists. Infantry branch officers accounted for at least half the enrolment and in the Twentieth Class for three-quarters of the student officers. Instructional materials in the 1930s included the superb tactical histories of the Russo-Japanese War, as the course concentrated on tactics. Students spent fewer hours on military history, studies and lectures on logistics, intelligence, airpower, and communications. After their two years of study they participated in a staff ride to the battlefields of the Russo-Japanese War in Manchuria and Korea, because these were the hypothetical battlegrounds of the next war.
Missing, however, was the dimension of a common education because of the conscious decision to limit special courses dealing with the operational level of war to a select ten officers annually. For them, Colonel Tani Hisao convened a special seminar using sensitive classified documents unavailable to other army officers let alone other War College students. Only a few War College faculty and students, for example, even knew that the compilation on the Russo-Japanese War titled Wartime Statistics existed for planning purposes. While the officially published version of the Russo-Japanese War stressed tactical fine points in illuminating detail, the crucial operational and administrative military history remained secret and hidden from most future staff officers. Thus most War College students argued tactics, and the ability to persevere in one's argument regardless of facts was highly valued. As one former instructor remembered, the ability, as the Japanese put it, to turn black into white was taken as a mark of confidence, because it displayed an officer's initiative and determination. Decibel levels became more important than knowledge in the classroom. Decisiveness and resolution were prized, perhaps because so much decision making in Japan requires a consensus that battlefield pressures did not permit.