The "Northern Expansion Doctrine" (北進論, Hokushin-ron or Northern Road) was a pre-World War II political doctrine of the Empire of Japan which stated that Manchuria and Siberia were Japan's sphere of interest and that the potential value to Japan for economic and territorial expansion in those areas was greater than elsewhere. Its supporters were sometimes called the Strike North Group. It enjoyed wide support within the Imperial Japanese Army during the interwar period, but was abandoned in 1939 after military defeat on the Mongolian front at the Battles of Khalkhin Gol (known in Japan as the Nomonhan incident). It was superseded by the diametrically-opposite rival policy, the "Southern Expansion Doctrine" (南進論, Nanshin-ron or Southern Road), which regarded Southeast Asia and the Pacific Islands as Japan's political and economic sphere of influence and aimed to acquire the resources of European colonies while neutralising the threat of Western military forces in the Pacific.
From the First Sino-Japanese War in the 1890s, Hokushin-ron came to dominate Japanese foreign policy. It guided both the Japanese invasion of Taiwan (1895) and the Japan–Korea Treaty of 1910 which annexed Korea to Japan. After the Russo-Japanese War (1904-5) Field Marshal Prince Yamagata Aritomo, a political and military ideological architect of Hokushin-ron, traced the lines of a defensive strategy against Russia. A February 1907 Imperial National Defence guideline envisioned two strategies: Nanshin Hokushin Ron (defence in the South and advance in the North) and Hokushu Nanshin Ron (defence in the North and advance in the South). There was intense discourse within Japan on the two diverging theories. Following World War I, Japanese troops were deployed as part of the Siberian Intervention during the Russian Civil War, with the hope that Japan could be freed from any future Russian threat by detaching Siberia and forming an independent buffer state. The Japanese troops remained until 1922, encouraging discussion by Japanese strategic planners of the idea of permanent Japanese occupation of Siberia east of Lake Baikal.
An essential step in the Hokushin-ron proposal was for Japan to seize control of Manchuria, so as to obtain an extensive de facto land border with the Soviet Union. Insubordination by rogue Japanese military personnel in the Kwantung Army in 1931 led to the Mukden Incident and provided a pretext for the Japanese invasion of Manchuria. As the Kwantung Army had 12,000 men available for the invasion of Manchuria at the time it needed reinforcements. War minister Sadao Araki was a solid supporter of the Hokushin-ron, and of a proposed attack on the Soviet Far East and Siberia. He arranged for Chōsen Army forces to be moved from Korea north into Manchuria without permission from Tokyo in support of the Kwantung Army. The plot to seize Manchuria proceeded as planned, and when presented by the fait accompli, all Prime Minister Reijirō Wakatsuki could do was weakly protest and resign with his cabinet. When the new cabinet was formed, Araki, as War Minister, was the real power in Japan. A puppet state was formed in Northeast China and Inner Mongolia. It was named Manchukuo and governed under a form of constitutional monarchy.
Hokushin-ron was largely supported by the Imperial Japanese Army. General Kenkichi Ueda was a strong believer in the Hokushin-ron policy, believing that Japan's main enemy was communism and that Japan's destiny lay in conquest of the natural resources of the sparely populated north Asian mainland. General Yukio Kasahara was also a major proponent of the Hokushin-ron philosophy, feeling strongly that the Soviet Union posed both a major threat and a major opportunity for Japan.
However, rival cliques of officers in the Army claimed to represent the "true will" of the Emperor. The radical ultranationalist Imperial Way Faction (Kōdōha) had many young activists who were strongly supportive of the Hokushin-ron strategy and a preemptive strike against the Soviet Union. They were opposed by the more moderate conservative Control Faction (Tōseiha), which favored a more cautious defence expansion and sought to impose greater discipline over the Army and war with China as a strategic imperative.
Relations between the Japanese Army and Navy were never cordial, and often marked by deep hostility, a situation whose origin can be traced back to the Meiji period. From the early 1930s the Army saw the Soviet Union as Japan's greatest threat and for the most part supported the Hokushin-ron concept that Japan's strategic interests were on the Asian continent. The Navy looked across the Pacific Ocean and saw the United States as the greatest threat, and for the most part supported the Nanshin-ron concept that Japan's strategic interests were in Southeast Asia and the Pacific islands. By the mid-1930s there was the serious possibility of a clash between the Army and Navy due to incompatible expansionist ideas.
The Kōdōha faction, which favoured Hokushin-ron, was dominant in the Army during Araki's tenure as Minister of War from 1931 to 1934, occupying most significant staff positions. However, many of its members were replaced by Tōseiha officers following Araki's resignation from ill health in 1934. In 1936, Kōdōha-affiliated young Army officers launched an unsuccessful coup d'état in the February 26 Incident. As a result, Kōdōha generals were purged from the Army, including Araki, who was forced to retire in March 1936.
The Imperial Defence Plan, formulated in June 1936, incorporated a balance of both Hokushin-ron and Nanshin-ron, requiring that both the Army and the Navy take a peaceful and unprovocative approach to their "enemies". The plan's goal was to acquire territories which possessed the raw materials, particularly petroleum, which Japan needed to sustain its growth and economy, but which it did not possess itself. Northward expansion (Hokushin-ron) would gain the natural resources of Siberia by attacking the Soviet Union via Manchuria. Southward expansion (Nanshin-ron) would involve seizing Indonesia and other colonies from the French, Dutch and/or British. Japan's supply of resources would eventually be assured by creating a "Greater East Asia Co-Prosperity Sphere". However, European powers had been dominant in Southeast Asia for more than a century, and Japanese foreign policy had little experience there. In pursuing Nanshin-ron Japan would risk – and in some quarters even welcome – a large-scale war with the great powers from across the globe.
In November 1936 the Anti-Comintern Pact was concluded between Japan and Nazi Germany. It agreed that in case of an attack by the Soviet Union against Germany or Japan, the two countries agreed to consult on what measures to take "to safeguard their common interests". They also agreed that neither of them would make any political treaties with the Soviet Union, and Germany also agreed to recognize Manchukuo.
A series of Soviet–Japanese border conflicts, without any formal declaration of war, began in 1932. Aggressive actions initiated by Japanese staff and field officers on the Soviet border with Manchukuo and Mongolia led to the disastrous Battles of Khalkhin Gol (1939) which resulted in heavy casualties for Kwantung Army and severely challenged its much-vaunted reputation. Any farther expansion northwards into Siberia was shown to be impossible given the Soviet superiority in numbers and armour. However, General Ueda continued to support the actions of his officers and refused to discourage them from taking similar actions, remaining adamant in his support of the Hokushin-ron policy. He was recalled back to Japan in late 1939 and forced into retirement. The Kwantung Army was purged of both its more insubordinate elements and its proponents of Hokushin-ron.
The Army lost prestige due to its failures in the Soviet–Japanese border conflicts; as a result the Navy gained the ascendency. It was supported in this by a number of the powerful industrial Zaibatsus, convinced that they could best serve their interests by fulfilling the needs of the Navy. The military setbacks on the Mongolian front, the ongoing Second Sino-Japanese War, and negative Western attitudes towards Japanese expansionist tendencies led to a shift towards Nanshin-ron in order to procure colonial resources in South East Asia and to neutralize the threat posed by Western military forces in the Pacific. Japan and the USSR signed the Soviet–Japanese Neutrality Pact in April 1941, freeing Japan for preparations for the Pacific War. When Nazi Germany launched its invasion of the Soviet Union in June 1941, Japan did not join its Axis ally's invasion by opening a second front in the Far East. Indeed, Japan did not militarily engage with the Soviet Union again until the USSR declared war on Japan in August 1945.