What-if: Imperial Japan Attacks the Soviet Union

EA Baker
1 day ago
10 min read

In the late 1930s and early 1940s, Japanese strategy teetered between two competing visions: expanding into the Pacific and Southeast Asia, or striking north against the Soviet Union. Following the defeat at Khalkhin Gol in 1939, Tokyo adopted the Nanshin-ron (“Strike South”) strategy, which ultimately led to the attack on Pearl Harbor, the conquest of Southeast Asia, and the outbreak of the Pacific War.

But what if the Hokushin-ron (“Strike North”) faction had prevailed? Suppose the Japanese had resolved their war in China, and the German invasion of the USSR succeeded far more dramatically, with Moscow and Stalingrad falling in 1942? With Siberia vulnerable, would Japan have risked a land war against the Soviet Union instead of striking south?

Two Military Branches, Two Competing Doctrines

Japan’s path to war vacillated between two competing doctrines. Its military had split into two powerful factions, each with its own institutional base, worldview, and enemies.

Hokushin-ron (Northern Road) was favored by the Imperial Japanese Army, especially the Kwantung Army stationed in Manchuria. Those in favor of this plan felt communism was the greatest existential threat. This meant Japan’s destiny was on the Asian continent. Control of Manchuria and Mongolia would anchor a continental empire and shield the Home Islands from Soviet pressure. Moreover, Siberia was viewed as a vital asset for timber, coal, iron ore, and even oil (particularly in northern Sakhalin), although much of Siberia's oil at this time remained undeveloped.

Map showing territorial expansions with black arrows over regions like Manchukuo, Sakhalin, and Outer Mongolia. Key years in red, seas in blue. — "Map illustrating Japan’s Hokushin-ron plans for a potential northern offensive against the Soviet Union. Dates indicate the year each territory came under Japanese control or influence.

Nanshin-ron (Southern Road) was the other plan, favored by the Imperial Japanese Navy. They felt the real threat to Japan was the United States and the European colonial powers that had dominated the Pacific. They felt that Japan’s security and prosperity depended on maritime expansion. Southeast Asia and the Pacific islands were lifelines rich in oil, rubber, tin, and other critical resources. The southern route pointed directly at the Dutch East Indies (now Indonesia), which was then one of the world’s major petroleum centers. Malaya’s rubber and tin would fuel the industry. Securing these shipping lanes meant they had to take the Philippines, which meant war with the United States of America.

Map of the Pacific during WWII (1937-1942), showing territories in blue for Japan and allies, and red for Allies. Arrows depict military movements. — Map showing Japanese expansion across the Asia-Pacific following the cancellation of the Kantokuen plan, highlighting territories acquired and occupied during the 1930s and early 1940s.

For the Navy, the Soviet frontier held little appeal. Siberia’s resources were distant, undeveloped, and landlocked. Control of the South Pacific islands would instead offer the resources they need, as well as offer a defensive perimeter around the Home Islands. Much of this was shaped by Mahanian sea power theory, emphasizing decisive fleet battles and control of sea lanes. Fear that Japan’s lack of oil (80% imported from the U.S. before 1941) made it fatally vulnerable unless it secured these southern supply areas.

Why Nanshin-ron Won Out

The Japanese Army’s defeat at Khalkhin Gol in 1939 dealt a serious blow to the Hokushin-ron faction. The campaign exposed the Imperial Japanese Army’s shortcomings in modern mechanized warfare. Japanese forces, still reliant on light tanks and limited transport, found themselves outmatched by the Red Army’s armor, artillery, and air power. The lesson was clear: a land war against the Soviet Union would be costly and uncertain.

This military humiliation compounded earlier political setbacks. The February 26 Incident of 1936, an attempted coup by radical young officers, had ended with the purge of many Army leaders who favored northern expansion. With their influence diminished, the “Strike North” doctrine lost its strongest advocates inside Tokyo’s military and political establishment.

At the same time, the lure of the South grew more urgent. The Pacific offered nearer and more accessible oil supplies in the Dutch East Indies, Malaya, and Borneo. By contrast, Siberia’s oil remained underdeveloped and far from Japan’s grasp, requiring massive investment and secure transportation routes, even if it were conquered.

The tipping point came with Japan’s occupation of French Indochina, intended to strangle China’s lifeline of foreign supplies. This move triggered sweeping U.S. sanctions, including the oil embargo of 1941 and the freezing of Japanese assets. Suddenly, the Navy’s case for southern expansion was unassailable: Japan needed oil, and the only way to get it was to strike south.

Yet, in what way could Imperial Japan have turned its sights north instead of south?

Assumptions for a “Strike North” Scenario

In this alternative scenario, Japan first secures a settlement or, at the very least, a truce in China. Such an outcome would free up a significant number of divisions tied down in the grueling conflict, allowing Tokyo to redirect manpower and resources toward new campaigns. Without the China war draining its strength, Japan would be in a stronger position to consider northern expansion.

Meanwhile, Germany would need to achieve greater success on the Eastern Front. If Moscow falls in late 1941, and by 1942, Stalingrad is encircled and captured, that would significantly alter the calculus. These victories leave the Soviet Union badly strained, its resources stretched to the breaking point. From Tokyo’s perspective, the opportunity to strike north into Siberia would appear far more promising under such conditions.

As such, the Strike South that launched the Pacific War, under this timeline, would need to be delayed or even avoided altogether. Japan would have to find a way to maintain neutrality with the United States. Their invasion of French Indochina really soured relations. Concluding the China war would resolve these tensions, yet it’s unclear in what manner Japan would agree to the terms. They had already poured in a lot of blood and treasure into the China quagmire and would most likely want terms the Chinese and their allies would reject. However, let’s assume a deal is established that brings the fighting to a close.

Finally, political unity emerges at home. The Army asserts dominance over strategic decision-making, ensuring that the Hokushin-ron doctrine prevails. With the Strike North faction fully in charge, Japan turns its gaze toward Siberia, ready to gamble on a land war against the Soviet Union rather than a naval clash with the West. So, what operational plans did the Japanese have in place for such an endeavor?

Kantokuen: Japan’s Secret War Plan

By early summer 1941, Japan’s Imperial Army was quietly formulating a bold, dangerous plan to attack the Soviet Union. This was Kantokuen (Kantōgun Tokubetsu Enshū —Kwantung Army Special Maneuvers), a plan to invade and occupy the Soviet Far East if the German invasion of the USSR provided a favorable distraction.

Kantokuen did not appear out of thin air. In fact, it was rooted in earlier Japanese contingency planning under Operational Plan No. 8, also known as Hachi-Go (“Plan 8”). Starting in 1938–1939, the Army General Staff and the Kwantung Army developed two variants, Concept A and Concept B, for a possible future war with the Soviet Union.

Map showing troop movements in Northeast Asia. Red arrows indicate army positions near major cities, with railways and seas labeled. — A map showing the Japanese initial offensive into the Soviet Far East, aiming ultimately to reach a line along the western slopes of the Greater Khingan Range

Here’s how they differed:

Concept A envisaged assaults from Manchuria’s eastern and northern borders, with defensive or holding actions in the west.
Concept B was far more audacious: pushing deeply westward from Manchuria, across the steppe toward Lake Baikal, to sever the Trans-Siberian Railway and cut off Soviet Far East forces from European Russia.

By mid-1941, after Germany invaded the USSR, the Strike North faction saw a window of opportunity. General Shin’ichi Tanaka, a notable proponent of the northern offensive, pushed to convert Hachi-Go’s theoretical constructs into an actionable plan. On 7 July 1941, Emperor Hirohito gave partial approval to the buildup. The plan was to move from readiness into offense in stages, once enough divisions, transport, and air power had been assembled.

Strategic Logic and Structure

Kantokuen was built on several assumptions that Japanese strategists believed might work in their favor. They felt that Germany’s campaign in the west would so strain the Soviet Union that it could not divert sufficient forces to the Far East. Upon taking control of the rail and port infrastructure in Manchuria and the Soviet Far East, they believed these facilities would support a rapid buildup of forces. Utilizing surprise, establishing air superiority, and the Japanese shorter lines of supply and movement compared to the USSR initially would give them an operational advantage. Lastly, there was a prevailing notion that the Soviets would be unable to mount large, coordinated counterattacks while fighting on two fronts (I don't think that's realistic, but that was the belief).

Kantokuen was designed around a multistage approach that included the following:

Build-Up & Readiness Phase

Japanese commanders would amass forces in Manchuria and Korea. The initial phase of the plan called for 23-24 divisions, tens of thousands of trucks, hundreds of tanks, hundreds of thousands of horses, and large numbers of laborers and logistical support to prepare for a major offensive.

Three-Phase Offensive

Once the forces were in place, the offensive would unfold rapidly. The aim was to isolate and destroy Soviet forces in pertinent border regions within six months. Key fronts included the Eastern Front (Primorye, Maritime Province), and secondary operations in more northern/western sectors.

Objective Lines

One objective was to reach a line along the western slopes of the Greater Khingan (Da Hinggan) Range, cutting off the Trans-Siberian Railway and severing Soviet reinforcements from Europe. Control of Vladivostok and Maritime Province was critical.

Order of Battle, Forces, and Logistics

Here is potentially how Japan’s armed forces would line up against the Soviet Union for this operation:

Category	Japanese Kantokuen (Planned)	Soviet Far East / Trans‑Baikal (1941)
Divisions	up to 50 (20–30 in main thrust)	23 (including 3 tank divisions, 2 mechanized divisions)
Personnel (includes support/labor forces)	~1,200,000	~650,000
Tanks / AFVs	~500	7,263
Artillery pieces	~3,000–4,000	15,187
Motor vehicles	~35,000 trucks + thousands of horses	57,329
Horses	~400,000	94,607
Aircraft	1,200–1,800	3,021
Front length	~5,000 km	N/A
Operational depth	up to 1,200 km (Lake Baikal)	N/A
Fortifications	minimal	11–50 fortified regions
Coastal artillery	minimal	up to 356 mm, ~150 pieces

These numbers need to be taken with a grain of salt, for many of them on the Japanese side were aspirational. I question whether they could’ve mustered such a force (1.2 million personnel), given that it would encompass the bulk of the Imperial Japanese Army’s force at this time. Garrison duty and other obligations would whittle down these numbers.

On the Soviet side, it’s difficult to know if we can truly trust these numbers, as the Soviet Army was known for manipulating records and falsifying information. The numbers we are looking at are also from mid-1941. With the invasion of Germany, these numbers might have changed as the USSR pulled forces to stop the German advance; however, there would still be a fear of the eastern threat from Japan, given the absence of a Soviet–Japanese Neutrality Pact in this timeline.

So, if the Japanese had managed to put themselves in a position to launch such an operation, how would it have gone?

Operational Weaknesses: Why Kantokuen Was Never Launched

The same assumptions that gave rise to Kantokuen also exposed its fatal flaws. Here are all the reasons why the operation was never launched:

Logistical Overreach: Japan lacked sufficient trucks, fuel, and roads to sustain large mechanized operations deep into Siberia. While the plan allocated tens of thousands of vehicles, actual vehicle strength and supply reserves were well short.
Infrastructure Constraints: Rail lines in Manchuria were overburdened; ports, supply depots, hospitals, and support infrastructure needed massive expansion. Without them, large force concentrations risked collapsing under their own weight.
Ambiguity of Strategic Priorities: Japan was already fighting a bitter war in China. The Navy and southern strategists were pushing hard due to the importance of oil, rubber, and the conflict with Western powers—simultaneous preparation for southern expansion distracted from northern readiness (assuming the Navy continues to look south and provides limited support to this army-dominated operation; we already mentioned that the China war would need to be resolved for them to undertake such an operation realistically).
Changing Geopolitics: As Germany’s progress in Europe slowed, the projection that Soviet resources would remain tied up in the west became less certain. The U.S. sanctions and oil embargo immediately pressured Japan, making delaying or shifting focus to the Pacific increasingly urgent. These external pressures helped draw attention away from Kantokuen. The diplomatic situation with the US would be crucial.
Imperial Reservations: Even after Emperor Hirohito approved parts of the plan, there were caveats. Tokyo made clear in some directives that the Kwantung Army was not to initiate offensive action unilaterally. Yet, as we’ve seen them do before, the Kwantung Army often did what it wanted due to its culture of gekokujō, "the bottom rules the high.” It describes the phenomenon of lower-ranking individuals, especially junior officers, disregarding and subverting the authority of their superiors and civilian leadership to seize power and push their own political agenda (just like it did during the Mukden and Marco Polo Bridge incidents).

With that being said, how would the Japanese have fared if they had launched such an operation?

Executing Kantokuen

If Kantokuen had been launched, it’s feasible that the Japanese would have achieved initial successes in border regions, likely capturing parts of Maritime Province, pushing toward Vladivostok, and striking Soviet rail lines (assuming they do not get bogged down by an unanticipated stiffer Soviet defense). As such, Soviet Far Eastern forces would be forced to commit resources to defending large swaths of territory, thereby limiting their ability to quickly reinforce against Germany. However, Japanese advances would most likely stall once supply lines stretched, Siberian weather worsened, or if the Soviets launched counterattacks using armor and reserves from west of Lake Baikal (which they would most certainly do).

In short, Kantokuen promised a potentially dramatic early burst of success, but sustaining momentum deep into Siberia was likely beyond Japan’s logistical and industrial capacities. After capturing those early gains, the Imperial Japanese Army may have to call on the Imperial Japanese Navy for help to hold what coastal gains they made amid Soviet counterattacks (something they would loathe to do). At the same time, the rest of the Imperial Japanese Army would have to hold the line against Soviet counterattacks. In such a precarious situation, they would have to hope that peace in the Pacific remains; otherwise, they may find themselves in a two-front war.

Final Thoughts

While a “Strike North” policy makes for a fascinating exploration into what could’ve been, it likely would have doomed Japan if Germany and its allies could not sustain its gains (and history showed us that is exactly what happened with Germany's land invasion into Russia). The resources that lured Tokyo south, oil, rubber, and tin, were not present in Siberia in usable form. The logistical challenges dwarfed Japan’s capabilities, and the Red Army, even under strain, could field mechanized counterattacks far beyond what the Chinese Nationalists ever could against Japanese forces.

As the strain of fighting on such a wide front begins to take its toll, would the Japanese resort to a desperate attack south to secure the resources they need to defeat the USSR? Especially if the Germans struggle to land the killing blow? Or would they desperately hold the line and hope their European allies would deliver them from disaster?

In either case, Japan’s success in such an operation would be predicated on whether:

It could fend off Soviet counterattacks without having to Strike South for resources to sustain the fight; and
How well its European allies fared on the other front.

In such a situation, I don’t think it would’ve ended well for the Empire of Japan.

If you enjoyed the blog, make sure to join my newsletter and never miss out on new content. Be sure to check out my extended editions of blogs like this one on my Patreon, linked below.